Home Secretary John Reid warned yesterday that fresh attacks on the UK were "highly likely" as he revealed that police were investigating around two dozen major terror plots.
As police continued to hold 23 men over airline liquid bomb allegations, Mr Reid said that at least four major terror plots had been foiled since last year's London bombings.
Amid anger over the handling of delays in British airports, Mr Reid said the Government had been responding to "extraordinary circumstances", with a plan to blow up multiple aircraft believed to have been imminent.
Britain was put on "critical" alert - the highest level of terror warning - early on Thursday morning as police launched raids in London, Birmingham and High Wycombe, Bucks.
Mr Reid told the BBC News 24 Sunday programme yesterday: "Even if on the basis of our intelligence ... we reduced from `critical' to `severe' ... it is highly likely there will be another terrorist attempt and that is one thing of which we can be sure."
Responding to a report in The Observer that "up to two dozen" terror probes were operating across the country, the Home Secretary said that there could also be unknown threats.
"I'm not going to confirm an exact number, but I wouldn't deny that that would indicate the number of major conspiracies that we are trying to look at," he said.
"There would be more which are not at the centre of our considerations and there may be more that we don't know about at all."
Asked how many terror plots capable of causing major loss of life had been foiled since July 7 last year, he said: "I can tell you that at least four major plots have been thwarted."
He also warned that despite the police operation, members of the alleged terror ring could still be "out there". "We believe it was a major, major plot," he said. "We believe ... that we have the main targets from that particular surveillance and plot, as I said there are still people out there who would carry out such attacks.
"We can't rule out either that there are people that we don't know about connected with this plot, or that there are others connected with other conspiracies, or indeed, that there are people who are prepared to use this opportunity to carry out a terrorist attack."
One of those arrested last week is set to appear in court tomorrow for a decision on extra detention while one has been released and a further 22 can be detained until Wednesday.
In Pakistan up to 17 others are believed to be in custody including Briton Rashid Rauf, who has been officially described as a "key" suspect and is thought to be the brother of Tayib Rauf, 22, one of those arrested in Birmingham last week.
Pakistan said the arrest of Rashid Rauf and others in the country had "triggered" Thursday's dawn raids in the UK.
PASSENGERS in the North-East are set for continued disruption today as part of security measures following a major anti-terrorist operation. Flights cancelled today from Newcastle are as follows:
* British Airways BA8070 07.05 departure to London Gatwick
* British Airways BA8074 09.50 departure to London Gatwick
* British Airways BA8076 14.40 departure to London Gatwick
* British Airways BA8078 19.30 departure to London Gatwick
* British Airways BA8073 09.20 arrival from London Gatwick
* British Airways BA8075 14.10 arrival from London Gatwick
* British Airways BA8077 19.00 arrival from London Gatwick
* British Airways BA8079 22.45 arrival from London Gatwick
* British Airways BA1327 12.05 departure to London Heathrow
* British Airways BA1335 17.45 departure to London Heathrow
* British Airways BA1326 10.45 arrival from London Heathrow
* British Airways BA1334 17.05 arrival from London Heathrow
* Information correct as of 8pm yesterday.