A woman is among those being questioned over the nationwide terror alert, it emerged today.
The 27-year-old was arrested with a 26-year-old man on the M6 last night and they are being interviewed at a central London police station, Scotland Yard said.
The development came as the country remained on its highest state of alert after three failed car bombings in London and Glasgow.
Airport yesterday to the discovery of two car bombs in the capital's West End on Friday.
Two men inside the Jeep were arrested at the airport, though one of them suffered severe burns after being engulfed in flames and is now in a critical condition in hospital.
Anti-terror officers from the Met and West Midlands Police later made the arrests on the M6 near Sandbach in Cheshire.
Today it was confirmed that a fifth arrest had been made, this time in Liverpool.
Scotland Yard said a 26-year-old man was held and two properties in the Liverpool area were being searched.
A large number of police descended on a house in a street just off Penny Lane in south Liverpool.
Officers were also searching a number of houses in the village of Houston, near Glasgow Airport, in connection with the attack there.
The Government yesterday raised the national terror threat level to "critical", its highest level, meaning an attack is expected "imminently".
Prime Minister Gordon Brown warned today that the threat to Britain was "long-term and sustained".
He said Britain's message to the terrorists must be, "We will not yield, we will not be intimidated and we will not allow anyone to undermine our British way of life."
In his first broadcast interview since becoming British PM on Wednesday, he said it was "clear" that the attacks in London and Glasgow were perpetrated by people who were associated with the global Islamist terror network al Qaida.
He urged the public to continue "living their lives as normal" and pledged, "Everything is being done in our power... to protect people's lives".
Home Secretary Jacqui Smith today chaired a meeting of the emergency contingencies committee Cobra, and told reporters outside the Home Office she had been updated on the latest security situation.
She thanked police and the emergency services for their "continued hard work" over the last 24 hours and confirmed she would make a statement to MPs in the House of Commons tomorrow.
The Jeep was driven into the main doors of the airport before bursting into flames at about 3.15pm yesterday.
Part of the Royal Alexandra Hospital in Paisley was evacuated yesterday after the arrival of one of the men arrested at the airport when a "suspect device" was found on his body.
But Strathclyde Police denied reports this device was a "suicide vest" after having it examined by explosives experts.
Meanwhile travellers started flying in and out of the airport again.
The first flights touched down almost 17 hours after the terrorist attack, shortly before 8am.
A spokesman for BAA Glasgow said 20 flights in and out had been cancelled so far today.
Liverpool John Lennon Airport was also closed overnight while police investigated a suspicious vehicle.
It was taken away for forensic examination and the airport was reopened at 4.40am, Merseyside Police said.
The rest of Britain's airports will stay open until further notice with a heightened police guard.
Most are bolstering security by closing off roads leading to the terminal buildings.
The Glasgow attack followed the discovery of two cars loaded with explosives in London's busy West End early on Friday morning.
The first, a Mercedes packed with a deadly cargo of petrol, gas canisters and nails, was found by paramedics called to the Tiger Tiger nightclub shortly before 2am after a person was taken ill.
The ambulance crew spotted smoke inside the car and immediately called police.