At least four major terror plots have been thwarted since the July 7 attacks here last year, it was revealed on Sunday as investigations into the conspiracy to blow up US-bound flights showed that terrorists were planning a wave of "apocalyptic" strikes on land and air.
The terrorist threat to Britain was still "substantial" despite the arrest of 23 suspects in connection with the foiled plot targeting airliners, British Home Secretary John Reid told the BBC's News 24.
All the four plots since the deadly bombings in July last year would have led to significant loss of lives and "up to two dozen" terror investigations were currently being pursued, he said.
Without giving details of the plots, Reid said the Government believed that the first Al-Qaeda plot in the UK was in 2000 in Birmingham, preceding the war in Iraq and the 9/11 bombings.
On Thursday, British police arrested 24 people for an alleged plot to blow up flights to the US emanating from UK. It is currently holding 23 of them, 17 of them with family ties in Pakistan.
"We think we have the main suspects in this particular plot. I have to be honest and say on the basis of what we know, there could be others out there... So the threat of a terrorist attack in the UK is still very substantial."
According to a report in 'The Independent' newspaper, suspected terrorists were planning to unleash a wave of "apocalyptic" attacks on land and air, using an arsenal of bombs and weaponry, including firearms, investigators have discovered.
The Independent quoted police and intelligence sources as indicating that the plot which was thwarted last week was targeted at the UK and the destruction of the airliners could have caused devastating loss of life and destruction on the British mainland.
Quoting a government source, the newspaper said "many dozens" of plots were under investigation, involving "hundreds" of suspects.
Security sources estimate that as many as 1,200 people here are actively involved with terrorism, and that the country is still under "very severe" threat from other potential plots.
Quoting sources, the newspaper said intelligence officers are aware of several active "jihadi" cells around the country including one in east London thought to be unconnected to the suspects arrested last week.
Investigators said surveillance in progress since the July 2005 London bombings had identified the locations of explosives and weapons in quantities sufficient to commit large-scale atrocities.
British Airways said it had scrapped 30 per cent of flights from Heathrow on the instructions of airport operator the British Airports Authority.
The ban on carrying liquids and gels onto airliners would continue indefinitely, raising questions about whether there are enough airport screeners to do the added work.
The restrictions are part of tighter airline security ordered by the Transportation Security Administration Thursday in the wake of the foiled terror plot.
The alleged conspirators planned to blow up as many as 10 planes flying from Britain to the US using liquid explosives, which the TSA's security equipment cannot detect in carry-on luggage.