The government raised the threat level to "critical" - the highest of five levels - on Thursday before arresting 24 people who were allegedly plotting to blow up US-bound passenger jets.

The threat level had been held at "severe" for about a year, after the tight security that immediately followed the July 7, 2005 attacks on London's transport network had been relaxed.

The "critical" alert status is designed to show that the country faces the threat of an imminent attack, whereas the "severe" alert status means an attack is highly likely.

The "critical" status triggers exceptional maximum protective security measures to meet specific threats and minimise vulnerability and risk.

Airlines warned

Meanwhile, London's Heathrow airport has threatened to ban airlines from the hub if they did not cut flights by 30 percent on Sunday, The Times said on Monday.

Airlines have been advised to cut
flights by 30  percent

Tony Douglas, chief executive of Heathrow, Europe's busiest airport, instructed all airlines with three or more flights scheduled for Sunday to show a schedule "demonstrating and  documenting a 30 percent reduction in departing passenger flights" in an email obtained by The Times.

Sent at 8pm (1900 GMT) on Saturday, the email continued: "Any airline not complying ... will be considered in breach of the Conditions of Use for Heathrow airport and the use of airport facilities will be denied."

British Airways also said on Sunday that it had "complied with a BAA directive to cancel 20 percent of its short haul operation tomorrow [Monday]".

It also cancelled all of its domestic flights from Gatwick, London's second-busiest airport.

A BAA spokesman told AFP earlier that the airport operator expected that between a fifth and a third of all departing flights from Heathrow would be cancelled on Monday.

He said BAA was "hoping [cancellations] will be less than 30  percent" on Monday. He added that cancellations "could be 20 percent, but probably not less".