A third airline has grounded a flight to the US for security reasons.
Continental Airlines acted for security reasons
Continental Airlines cancelled Sunday's 1215 GMT flight 17 from Glasgow to Los Angeles via Newark, New Jersey.
British Airways and Air France have also called off five US-bound flights on Sunday and Monday, amid fears al-Qaeda may be targeting them.
A US Department of Homeland Security spokesman said: "We remain concerned about al-Qaeda's desire to target international aviation."
He added: "There was specific credible threat information that was shared
with some foreign governments, including the British and French
Governments, and the decisions were made to cancel these flights."
US officials believe al-Qaeda is again trying to target international flights.
But neither the airlines nor the UK Government has commented on this.
Continental Airlines flight 17 Glasgow-LA 1 Feb
BA flight 203 London-Miami 1 Feb
BA flight 223 London-Washington 1 Feb & 2 Feb
BA flight 222 Washington-London 1 Feb & 2 Feb
Air France AF026 Paris-Washington 1 Feb & 2 Feb
Air France AF207 Washington-Paris 1 Feb & 2 Feb
American media quoted sources as saying the threat was new information from an informant, which was corroborated by other sources who named specific flights.
Continental spokesman David Messing said the Glasgow-Los
Angeles flight was cancelled "because we were unable to obtain the necessary security
clearance from the Department of Homeland Security and its international
Air France flights AF026 were cancelled on Sunday and Monday
"for reasons of security," spokeswoman Veronique Brachet
A BA spokeswoman said: "The safety and security of our operations is our
absolute priority and will not be compromised."
BA 223 was the subject of concern early in
January, when it was cancelled twice because of security
fears and then delayed for hours several more times.
Defence analyst Paul Beaver said: "People in Washington are saying that this is basically good, straightforward intelligence about flight numbers and destinations.
"Between the first of February and somewhere around the 21st or 23rd of February, there seems to be a period where there is a heightened risk."
Mr Beaver said al-Qaeda was changing tactics, posing a continuing problem for authorities.
"Al-Qaeda adapts and evolves. What they are after now is American, British and European passport-holders.
"They are after clean-looking people, business people or holiday travellers who appear to be smartly dressed.
"In other words, they are trying to put off the intelligence services."
Paul Wilkinson, professor of international relations at the University of St Andrews, said security agencies must have had evidence of a specific threat to cancel the flights.
He said: "Flight cancellations are exceptional measures. America is convinced there was a threat to flights bound for
the US in early January - that threat has not been eradicated."
A British Department for Transport spokesman said the
decision to cancel the flights was made "in the light of
He said: "Aviation security measures are adjusted from time to time, and occasional cancellations may be necessary.
"The first priority is always the safety of the travelling public."