First Minister Peter Robinson is recovering in hospital after undergoing a procedure following a suspected heart attack.
A statement issued by the DUP said the 66-year-old felt unwell this morning.
A spokesman said: "The First Minister felt unwell this morning and has been admitted to the hospital for some further tests."
A senior DUP source has said it is understood it is a heart related condition.
Mr Robinson was taken to the Ulster Hospital, Dundonald just after 9am this morning and was transferred to the Royal Victoria Hospital, Belfast.
The Royal is Northern Ireland's largest hospital and the main centre for cardiology.
A statement from the hospital said: "Mr Robinson underwent a procedure this morning and is currently recovering in the RVH.
"He and his family have requested the need for privacy from this point onwards."
Politicians from across the political spectrum have wished Mr Robinson a speedy recovery.
Among those was Prime Minister David Cameron who tweeted his well-wishes.
He said: "My best wishes to Northern Ireland's First Minister, Peter Robinson who is in hospital. I hope he has a speedy recovery."
Deputy First Minister Martin McGuinness was among several other politicians who took to Twitter to wish Mr Robinson a speedy recovery.
The Sinn Fein MLA said he was "concerned" to hear Mr Robinson was in hospital.
My best wishes to Northern Ireland's First Minister, Peter Robinson, who is in hospital. I hope he has a speedy recovery.— David Cameron (@David_Cameron) May 25, 2015
"Concerned to hear First Minister Peter Robinson has been admitted to hospital. My thoughts and prayers are with him, Iris and family."
The Ulster Unionist Party leader Mike Nesbitt wished Mr Robinson a "speedy recovery".
Mr Nesbitt said: "I am sorry to hear that the First Minister has been taken into hospital this morning and I wish Peter a full and speedy recovery. Our thoughts are with him and his family."
Concerned to hear First Minister Peter Robinson has been admitted to hospital.My thoughts & prayers are with him,Iris & family.— Martin McGuinness (@M_McGuinness_SF) May 25, 2015
DUP deputy leader Nigel Dodds said: "Best wishes to Peter Robinson for a quick and full recovery, All our thoughts and prayers are with Peter and family. God Bless."
Former DUP health minister Jim Wells said he could not recall Mr Robinson having a sick day in 40 years.
He said: "I have known Peter since 1975 and I cannot remember him ever having a day off sick.
On behalf of the Ulster Unionist Party I wish Peter Robinson a full and speedy recovery from the issue that has seen him hospitalised— Mike Nesbitt (@mikenesbittni) May 25, 2015
"He always struck me as being fit and healthy so, it was a real surprise to hear the news.
"The party will be 100% behind me in wishing him well. We hope he will be back on his feet as soon as possible.
"It will be strange not having him with us at this crucial vote tomorrow - assuming he won't be there. I wouldn't put it past him to bounce back quickly."
Peter Robinson in hospital. I hope he is ok.— Gerry Adams (@GerryAdamsSF) May 25, 2015
Sinn Fein president Gerry Adams was among others taking to social media to wish the First Minister well.
"Peter Robinson in hospital. I hope he is ok," Mr Adams said.
SDLP leader Alasdair McDonnell said he was sorry to hear the news.
Best wishes for a speedy recovery to Northern Ireland's First Minister, Peter Robinson http://t.co/kOE3vXUrxk— Nicola Sturgeon (@NicolaSturgeon) May 25, 2015
He said: "I am very sorry to hear that Northern Ireland’s First Minister Peter Robinson has been taken into hospital this morning.
"I would like to extend the thoughts, prayers and best wishes of both myself and my SDLP colleagues to Peter Robinson and his family at this stressful time. I wish Peter a full and speedy recovery."
Alliance Party leader David Ford said: "On behalf of the Alliance Party I would like to send Peter Robinson my best wishes. I hope he will be able to make a speedy recovery."
The Traditional Unionist Voice leader Jim Allister said: "I was sorry to hear that Peter Robinson was admitted to hospital this morning.
“My thoughts and prayers are with him and his family at this concerning time.”
First Minister of Scotland Nicola Sturgeon also tweeted her best wishes for a "speedy recovery" for Mr Robinson.
Meanwhile Presbyterian Moderator Dr Michael Barry said it was a "worrying time" for the First Minister's family.
The US government has also wished Mr Robinson a swift return to good health.
Former US senator Gary Hart, who hosted talks with the five Executive parties in the run up to December's Stormont House Agreement, said he had been concerned by the development.
In a statement, he said: "I and my colleagues in the US government were very concerned to learn that Northern Ireland's First Minister Peter Robinson was admitted to hospital earlier today.
"Our thoughts are with him and his family and we wish him a prompt return to full health."
Peter Robinson, who has three children, was a founding member of the DUP.
Born in Belfast just after the Second World War, and schooled at Annadale Grammar in the south of the city, he was inspired by the firebrand oratory of the Rev Ian Paisley and joined him in his battle to protect Ulster from the hands of republicans.
In 1985 he was at the forefront of the pan-unionist campaign against the Anglo Irish Agreement - an accord between the British and Irish governments which gave Dublin a role in affairs in Northern Ireland.
In his latter career he has been a trenchant opponent of all forms of paramilitarism, but in those dark days of the Troubles the lines were somewhat blurred having been arrested in the village of Clontibret, in the Irish Republic, after taking part in a so-called "invasion" along with 500 loyalists as part of the protest.
He later pleaded guilty to unlawful assembly.
Mr Robinson and his party colleagues opposed the 1998 Good Friday Agreement, citing its release of paramilitary prisoners as abhorrent.
Atlhough he took office as minister for regional development in the newly created power sharing executive, he refused to take part in cabinet meetings.
The eventual collapse of that assembly benefited the DUP at the polls and they soon displaced the Ulster Unionists as the largest unionist party in Northern Ireland and, in 2006 negotiated the St Andrews Agreement which paved the way for the latest assembly.
To all intents and purposes, Peter and Iris Robinson became Northern Ireland's "First Couple" when he realised his long term ambition and became Stormont First Minister and DUP leader after the retirement of Ian Paisley in 2008.
Mr Robinson served as MP for East Belfast from 1979 until 2010, when a shock result saw Alliance's Naomi Long win the seat.
Mr Robinson's period as head of the DUP has been characterised by mixed relations with his coalition partners Sinn Fein.
In a significant development for community relations in Northern Ireland in 2012, he joined deputy First Minister Martin McGuinness at a Gaelic Athletic Association (GAA) cup final.
Gaelic games are associated with the nationalist community, but his attendance at the Armagh Athletic Grounds follows long-running efforts to build bridges between the GAA and the unionist community.
He has bolstered his traditional unionist credentials by forming a pact ahead of the general election by the Ulster Unionists to maximise the number of unionist seats and has made a resolution of the impasse over loyal order parades one of his priorities.
But his most immediate challenge had been expected to be this coming week when an expected Sinn Fein veto of welfare reform legislation will leave a gaping hole in the powersharing administration's budget and could precipitate its collapse.
Peter and his wife Iris met at Cregagh Technical College in east Belfast when she was 16.
“He was very handsome, he stood out from the other boys,” she recalled in a 2008 interview.
In this same interview with the Sunday Tribune Mrs Robinson spoke frankly about long-standing rumours that her husband had beaten her.
“This malicious lie was started by the Government in an attempt to blacken Peter's name when he was protesting at the Anglo-Irish Agreement.
“It took root because I was in hospital 17 times during that period with gynaecological problems,” she said.
Her involvement in frontline DUP politics began alongside her MP husband as a fellow Castlereagh councillor from the 1980s.
But she soon established a reputation as a major electoral force in her own right, in no small part due to her record of constituency work.
Strangford — next to her husband's East Belfast base — was steadily turned into DUP territory. Her victory there in the 2001 General Election was one of the key steps in the party's march to the top of unionism.
Mrs Robinson was dubbed the unofficial First Lady of Northern Ireland when her husband took the top Stormont job in 2008.
However, Mrs Robinson, a born-again Christian, sparked outrage and a police investigation when she described homosexuality as an abomination and claimed gay people could be "cured".
In 2010 the DUP suffered considerable damage in the extraordinary political firestorm which followed Iris's admission of an affair with a teenager.
She was expelled from the party and was said to be receiving "acute psychiatric treatment".
Last year Ian Paisley (snr) launched an explosive attack on his former colleagues in a BBC documentary, where he alleged Mr Robinson was part of a plot to oust him as DUP leader -- a claim Mr Robinson denied.
An internal survey was alleged to have been compiled on Mr Paisley's leadership.
Asked what it was about, he replies: "Getting rid of Ian Paisley", adding that it had been carried out "in the interests of the people who took over".
He is asked if that included Mr Robinson, replying: "Oh yes, he would have been."
Mr Paisley also said he could no longer have the same relationship with Mr Robinson, adding: "His ways are not my ways."
Mr Robinson responded: "As someone who faithfully served Dr Paisley for many decades, I will make one final sacrifice by not responding and causing any further damage to his legacy beyond that which he has done himself. Rather than return insult for insult, let me bless him with the mercy of my silence and wish him well."
Ian Paisley died on 12 September last year.
Mr Robinson threatened to resign as First Minister in February 2014 unless there was a judicial inquiry into the on-the-runs scandal, in which secret "amnesty" letters were given to more than 180 IRA suspects.
Prime Minister David Cameron caved into pressure and promised an inquiry hours before a midnight deadline set by Mr Robinson– pulling Northern Ireland back from the brink of political meltdown.
The DUP leader's ultimatum followed the collapse of the trial of John Downey, suspected of involvement in the 1982 Hyde Park bombing which killed four soldiers.
Additional reporting PA