BA's flight 223 to Washington was delayed for the third day running on Monday, amid security fears.
About 200 passengers endured the prolonged wait
The flight, which was cancelled twice last week and delayed on Saturday and Sunday, had been due to take off from Heathrow at 1505 GMT.
But it was not cleared by US officials until almost three hours later, and finally took off at 1818 GMT.
However, BA flight 263 to Riyadh, also cancelled twice last week, left more or less as scheduled at about 1345 GMT.
The delays were reportedly caused by US officials screening all 200 or so passengers on board.
"We are required to send data to the US and were waiting to hear back from
them before the flight could go," a BA spokeswoman said.
At the same time, a row continued about US demands that undercover armed sky marshals be put on some flights.
Foreign Secretary Jack Straw defended the decision to ground some passenger flights, saying safety must come first.
Transport Secretary Alistair Darling said the flights were grounded only as a last resort.
However, neither minister would be drawn on the specific nature of any potential threat.
The pilots' union Balpa claimed the repeated grounding of 223 was a "political decision" linked to BA's reluctance to fly with sky marshals on board.
The flight was accompanied by US fighter jets last Wednesday (New Year's Eve), cancelled on Thursday and Friday and subject to delays on Saturday and Sunday.
Speaking on BBC Radio 4's Today programme, Mr Straw said the cancellations had been based on specific intelligence.
He denied that UK air security - including the decision to introduce air marshals on some planes - was being driven by the US.
"We make our own judgements on the basis of expert analysis by our brilliant security and intelligence services," Mr Straw said.
Some pilots believe the delays were all about air marshals
On Monday UK holiday company Thomas Cook Airlines joined the row over air marshals, saying it would refuse to operate
flights if they were put on board because of safety concerns.
"For the time being, if a sky marshal
presents himself, we will cancel the flight," said a spokesman.
"Like Balpa, we want to see captains
remain in full control of the aircraft at all times."
Balpa officials met BA to discuss the issue on Monday, and were due to meet Mr Darling on Tuesday.
The union said good progress had been made in the talks with BA, and further talks would be held.
Balpa has already reached agreement with one transatlantic carrier - Virgin
Atlantic - on rules for working with the marshals.
It is calling for an "over-arching" agreement with the
government, covering the use of marshals across all airlines.