British Airways cancelled two flights to Washington because of fears that a terrorist suspect was planning to board the aircraft, the BBC has learned.
Security has been stepped up at Heathrow
The report has not been confirmed, but it is thought terrorists could have been planning to hijack the plane.
The BA223 flight from Heathrow was grounded for the second day running on Friday and hours later, BA said it had cancelled a flight to Saudi Arabia.
The airline is consulting ministers over Saturday's Washington flight.
A British Airways spokeswoman said the airline was still hoping to run the 1505 GMT service, and a decision on whether to fly would be made on Saturday morning.
BBC security correspondent Frank Gardner said the Washington flights on Thursday and Friday had been grounded because a terrorist suspect known to US authorities was believed to be trying to board it on both days, according to a source.
The suspect was thought to be attempting to hijack the aircraft and crash it into a target in Washington, but he did not turn up at the airport, said the source.
In a separate development, a source told the US network ABC that someone outside the US had supplied information to American authorities about a plan to hijack flight 223.
American security officials are trying to establish how reliable the information is.
The assistant director of the FBI in Washington, Michael Mason, acknowledged the flights were being cancelled in response to a specific threat - what he described as coded talk.
Transport Secretary Alistair Darling told BBC Radio 4's Today programme tighter security may mean some inconvenience for air passengers.
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
Saturday - BA flight 263 from London to Riyadh cancelled
"The threat that we are now facing is going to endure for many years," he warned.
He said: "There may be a need to ground particular flights because of specific information we have got."
But he said this did not mean there would be wholesale cancellation of flights.
On Wednesday Flight BA223 was held on the tarmac in the US capital while agents questioned passengers.
In a separate development on Friday, BA cancelled flight 263 which had been due to leave for the Saudi capital Riyadh from London at 1330 GMT on Saturday.
Its return flight on Sunday has also been cancelled.
BA also said it had cancelled a flight to Riyadh on Wednesday and its return flight on Saturday, and was reviewing whether to fly there on Monday.
The decisions on both the Riyadh and Washington flights are thought to have followed advice from the UK Government, prompted by US intelligence.
Passengers had been checking in for their trips to the US capital on Friday when BA said the plane would not be taking off.
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.
Friday's 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.
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.