British Airways has cancelled a flight to Washington for the second day running, amid fears of a terror attack.
The BA flight has been under the spotlight for three days
The decision to ground flight BA223 was "due to security reasons" and followed government advice, the airline said.
Passengers had been checking in for their trips to the US capital when BA said the plane would not be taking off.
Following intelligence from the US the same flight had been grounded on Thursday, and on New Year's Eve it was given an escort by fighter planes.
Asked what kind of security alert led to the 1505 GMT flight being cancelled, BA said: "It is that particular flight that there are issues with, not the passengers."
William Mallett, 38, said he was "nervous" despite extra security
A spokesman told BBC News Online on Friday that the flight was cancelled "following the latest advice from the UK Government received this afternoon".
BA hoped to offer the 300 passengers alternative flights - but some were left feeling uneasy.
Among them, Deepa Menon, 28, a law student from Washington DC, said: "I am irritated.
"I am sure there are reasons - but I do wish we had known what was going on earlier."
And Svenja Steinfelder, 29, a German research fellow studying in Washington, said the scares had left her feeling "very nervous" about flying.
The early evening BA flight to Washington - the BA225 - took off 40 minutes later than its scheduled departure time of 1840 GMT after taking some of the passengers from the grounded BA223.
Defence analyst Paul Beaver said the alert was based on "very high grade intelligence" and suggested some kind of attack was being planned.
Flights hit by security fears
Christmas - six Paris to Los Angeles flights cancelled
Wednesday - AeroMexico flight from Mexico City to Los Angeles cancelled
Wednesday - BA flight 223 shadowed by fighter jets, plane searched and passengers questioned
Thursday - BA flight 223 from London to Washington cancelled
Thursday - Air France New York flight lands in Canada for baggage check
Thursday - BA flight from Washington delayed for extra security checks
Friday - BA flight 223 from London to Washington cancelled
He said it was possible al-Qaeda or a sympathetic group based in Europe planned to hijack the plane and crash it into a building in the US or blow it up.
BBC correspondent Andy Tighe said BA bosses had been in meetings since 0900 GMT, so the late cancellation suggested "there was something happening at the last minute".
On New Year's Eve the same flight was escorted in to Washington by fighter jets, with officials searching the plane and questioning passengers for hours.
It was not allowed to fly at all on Thursday and whether or not the flight will also be cancelled on Saturday remains unclear.
Later on Friday it emerged Home Secretary David Blunkett had held phone talks with US Homeland Security Secretary Tom Ridge to discuss ways to minimise disruption on flights to the US.
America recently tightened security after intelligence warnings led its terror alert status to be raised to orange - the second highest level available.
It announced that certain flights will not be able to fly to or from the country, or through its airspace, unless air marshals are on board.
A Mexican flight to the US was also cancelled on Wednesday amid security fears.
In December six Air France flights from Paris to Los Angeles were grounded, at the request of the US.
The FBI said on Friday that passengers
wrongly identified as potential security threats had led to the
cancellation of the flights.
The French Government believes the mix-up was partially down to the mistaken translation of some Arabic names.
Meanwhile, a Chilean man was arrested on an Easyjet flight from Madrid to Luton on Thursday, when he was found to be in possession of two pen-knives shortly before take-off. The man was said to be "making references to the end of the world". The flight took off 75 minutes late.