British Airways flight 223 to Washington has been delayed at Heathrow for the fourth day running.
About 200 passengers endured the prolonged wait on Monday
The flight, which was cancelled twice last week and delayed on Saturday, Sunday and Monday, was due to take off at 1505 GMT on Tuesday.
But it did not take off until about two hours later, as BA reportedly waited for clearance from the US authorities.
Foreign Secretary Jack Straw has previously said the cancellations were in response to "specific intelligence".
The delays on the subsequent flights 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 on Monday.
The delays to flight 223 come amid a growing row about continued US demands to introduce armed sky marshals on flights.
Pilots' union Balpa has claimed the repeated grounding of the flight was a "political decision" linked to BA's reluctance to fly with sky marshals on board.
But Mr Straw has 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.
Balpa representatives discussed the issue of marshals with Transport Secretary Alistair Darling on Tuesday.
Balpa general secretary Jim McAuslan described the meeting as "very useful", and said Mr Darling had agreed to work towards an industry-wide agreement about how the marshals would be deployed.
Before that was developed, he said, the union's advice remained that its members should not fly with a marshal on board.