British home secretary Jacqui Smith will make an emergency statement to MPs in the Commons this afternoon amid growing police confidence that they will identify the network behind the latest terror attacks on London and Glasgow.
The UK's official threat assessment was raised to its highest level - critical - on Saturday night as Scottish police directly linked the burning car attack on Glasgow airport with the discovery of two car bombs in London on Friday.
Authorities evacuated a terminal at Heathrow airport last night to investigate a suspect package but later reopened it after declaring the area safe.
The cabinet's emergency Cobra committee met for a fourth time yesterday as prime minister Gordon Brown warned Britons they faced "a long-term and sustained threat".
At the same time the head of Scotland Yard's anti-terrorism command, deputy assistant commissioner Peter Clarke, said he was "absolutely confident" of uncovering the attackers' methods and network. Mr Clarke said that forensic searches of all three vehicles and the examination of thousands of hours of CCTV footage were proving "extremely valuable" in the search for the perpetrators of the attacks, for which there was no advance intelligence warning.
As police searched premises near Glasgow and in Liverpool and Staffordshire, five people were in custody last night - one man was arrested in Liverpool and a man and woman detained in Cheshire, in addition to the two men arrested after crashing the burning green Jeep Cherokee into the main terminal at Glasgow airport on Saturday afternoon.
One of these two suspects was under armed guard at the Royal Alexandra Hospital in Paisley where he was taken suffering from severe burns. The suspects in custody are not believed to be British and at least one other suspect is still at large, it was reported last night.
Flights were resumed at the airport yesterday as Strathclyde Police appealed for information about sightings of the vehicle in recent days, while Scottish First Minister Alex Salmond said it appeared the people involved in the Glasgow incident had not been in Scotland "for any length of time".
The move to the top-level "critical" threat assessment confirmed the authorities' fears of further "imminent" attacks, as security was stepped up at airports across the UK, including Edinburgh, Belfast, Newcastle, Birmingham, Liverpool, Manchester and Blackpool.
There was also heightened security at yesterday's Princess Diana memorial concert at Wembley and it will be tight in southwest London today as Wimbledon resumes.
Ms Smith will update MPs on the police, security and intelligence investigations into the failed car bombings, while signalling government attempts to build a fresh cross-party consensus for a new package of anti-terror laws. The Conservatives said they were prepared to listen if the government produced "convincing evidence" in support of its case for extending the period allowed for holding terrorist suspects from 28 to 90 days.
A review now seems certain to result in the admission of wiretap evidence in the courts, and the Conservatives are also prepared to support changes to allow police resume questioning suspects after they have been charged with an offence.
In his first major interview since becoming prime minister, Mr Brown pledged to do everything necessary to defend the British people while promising to "balance" security measures with protection for civil liberties.
Mr Brown said it was clear they were dealing in general terms "with people who are associated with al- Qaeda". He told the BBC it was obvious there was a group of people prepared to inflict "maximum damage on civilians".
However, he added: "We've got to separate those great moderate members of our community from a few extremists who wish to practise both violence and inflict maximum loss of life in the interests of a perversion of their religion."