The SDLP has ruled out a "nakedly sectarian" nationalist electoral pact with Sinn Fein in Northern Ireland during the general election.
The Democratic Unionist Party (DUP) and the Ulster Unionist Party (UUP) have agreed to field one unity candidate in each of four general election constituencies in one of the most far-reaching eve-of-poll deals for decades.
East Belfast, North Belfast and the border constituencies of Newry & Armagh and Fermanagh & South Tyrone will be covered.
Party strategists aim to increase the number of unionists at Westminster with the odds of a hung parliament and increased influence for the smaller parties high.
SDLP leader Dr Alasdair McDonnell said: "The SDLP is absolutely opposed to nakedly sectarian electoral pacts.
"We have consistently argued against such pacts and refuse to put party political interests ahead of the need for reconciliation in our divided society.
"I am deeply disappointed that the unionist parties have pressed ahead in this shamefully divisive way."
Sinn Fein had called on nationalists to come together to support progressive politics in response to the unionists' "narrow sectarian and conservative agenda" for the May election.
DUP leader Peter Robinson said: " This is the most comprehensive electoral agreement between our two parties in the last 29 years."
He said he wanted to maximise the pro-Union voice in the House of Commons.
For months many have been predicting that not only will neither the Conservatives nor Labour command an overall majority in the Commons or form a coalition with one of the smaller parties, i t also appears that one or other of them will have to assemble some form of patchwork alliance involving multiple parties, or run as a minority administration with no guarantee it can get its business through the House.
The DUP, the largest Northern Ireland party in the Commons, has said it will support a government in a hung parliament if there is agreement to scrap the so-called bedroom tax, commitments on defence spending and securing UK borders.
Unionists face a challenge at the polls from Jim Allister's hardline Traditional Unionist Voice, which opposes Sinn Fein's inclusion in power-sharing at Stormont.
Ukip is to run a series of candidates in Northern Ireland which may also erode the mainstream unionist vote.
In East Belfast, the DUP and UUP will encourage support for Gavin Robinson, a former Belfast City Council lord mayor.
Peter Robinson lost his seat in the last election to Alliance's Naomi Long by around 1,500 votes.
She said: "The DUP clearly now know that Gavin Robinson cannot win against Alliance in a fair fight, so they have resorted to this anti-democratic move in a desperate attempt to strengthen their position."
In Fermanagh & South Tyrone, both parties will encourage support for senior Ulster Unionist Tom Elliott.
Sinn Fein's Michelle Gildernew took the seat by four votes last time after another unionist unity candidate ran against her.
In North Belfast, both parties will encourage support for incumbent Nigel Dodds, deputy leader of the DUP.
In 2010 in North Belfast, Mr Dodds polled 14,812 votes, with Sinn Fein's Gerry Kelly on 12,588.
In Newry & Armagh, both parties will encourage support for Ulster Unionist Danny Kennedy, Stormont's regional development minister.
The incumbent is Sinn Fein's Conor Murphy.
Peter Robinson has said it had not been possible to include the constituencies of Upper Bann and South Belfast in the pact.
Dr McDonnell is the incumbent in South Belfast while the DUP's David Simpson holds Upper Bann.
Mr Murphy said the SDLP should put progressive politics first.
"The coming together of the two main unionist parties, the DUP and the UUP, in an election pact on a narrow sectarian and conservative agenda is a challenge to progressive politics," he said.
"These parties have no vision for the future and are happy with crumbs from the Westminster table."
He added: "Nationalists, republicans and other progressives should come together to ensure maximum representation for parties committed to defending the core public services of health, education and the welfare state, parties which are unequivocally for equality and which are totally opposed to sectarianism, racism and homophobia."