Air passengers can expect delays caused by security alerts for years to come, says Transport Secretary Alistair Darling.
His warning came after a British Airways flight, cancelled for two days and delayed by an unknown threat, landed in Washington.
He would not discuss newspaper reports that UK intelligence believed al-Qaeda was planning to use BA flights for suicide attacks on the US.
Mr Darling said: "I fear that for many years to come we are going to be living in an age where there is going to be a heightened state of alert.
"Sometimes it will be quite severe, at other times perhaps less so.
"
On the whole, there were few signs of fear or even apprehension
"
Andy Tighe
BBC correspondent
"We are going to have to get used to increased security at airports. From time to time that will be noticeable and at other times maybe things will be going on behind the scenes."
This increased vigilance was evident on Sunday when flight BA 223 from Heathrow to Washington DC was again delayed for security checks.
The same numbered flight was cancelled on Thursday and Friday and delayed on Saturday before departure amid security concerns.
A BA spokesman said the US had asked for extra information about the flight "for security purposes" before the scheduled take-off at 1505 GMT, as had been the case on Saturday.
Mr Darling - speaking on BBC One's Breakfast with Frost programme - did not talk about what specifically led to the previous grounding of flight 223, and the cancellation of weekend BA flights to and from Riyadh in Saudi Arabia.
But he stressed the decisions were "justified" on the basis of intelligence warnings.
Cancellations 'rare'
Such information arrived daily, the transport secretary went on, and was evaluated by intelligence experts before being discussed by ministers.
It was often sketchy, he said, but added: "At the end of the day, we have got to reach a judgement based on the information we have as to whether, in extreme cases, a flight has to be cancelled or whether other measures would be adequate."
Mr Darling stressed that cancellations were "comparatively rare", and added that British Airways had taken the final decisions about its recently cancelled flights based on government advice.
FLIGHT SECURITY FEARS
The airline's flight to Washington on Saturday eventually landed without incident after being grounded on Thursday and Friday and delayed for several hours for extra security checks before take-off.
BBC security correspondent Frank Gardner said his sources believed there were fears a terrorist suspect was planning to hijack the aircraft and crash it into a target in Washington.
An official with the US homeland security department confirmed there had been "very specific and credible concerns" about the flights.
But there were separate reports in the New York Times, quoting an American official, that the BA flights had been grounded because of pilots' reservations about flying with air marshals on board.
On Sunday the Observer newspaper said an internal BA memo showed deep opposition among executives to the government initiative.
BA's operations director, Mike Street, had hinted the airline would refuse to fly with air marshals on board because it would mean there was a threat to passengers, the paper said.
The Observer also reported fears that terror suspects thought to be planning an attack on BA flights may hold legitimate UK, US or other European passports.
It claims new evidence suggests extremists are trying to recruit suicide bombers from educated classes in the UK, because they can more easily penetrate tight security.
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