British Airways and Air France have cancelled a number of US flights amid fears al-Qaeda may be targeting them.
BA also cancelled several flights in January over security fears
Flights BA 223 from London Heathrow to Washington on Sunday and Monday are hit as well as return flights BA 222.
A BA flight to Miami on Sunday was also grounded on security advice, as were four Air France flights.
The US Department of Homeland Security said that it remained "concerned about al-Qaeda's desire to target aviation, especially international aviation".
"The US intelligence community continues to gather specific credible threat information on international flights," it added.
US officials said they had received reports of "al-Qaeda's desire to target" five or six specific flights from Europe, Reuters news agency reported.
A BA spokeswoman said safety was the airline's top priority.
"The safety and security of our operations is our
absolute priority and will not be compromised," she said.
Air France flights AF026 were cancelled Sunday and Monday
"for reasons of security," spokeswoman Veronique Brachet
The return flights on Sunday and Monday from Washington to Paris - AF027 - were also cancelled, the AFP news agency reported.
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."
Mustafa Alani, from the Royal United Services Institute, said the information could come from two sources.
"We have the human intelligence, basically an informer or a group that passes on such information, or we have electronic intelligence, the chatter and internet exchange between al-Qaeda groups and network."
A third airline was also under threat, according to Associated Press who quoted an unidentified US government official.
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
The official refused to name the US-based carrier which flies international routes.
BA's Flight 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.
US officials said on Friday new intelligence indicated BA
Flight 223 and Air France flights from Paris to an
unspecified US city could be terror targets.
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."
The US Homeland Security Department has said it has no plans to raise its current alert level.