Airlines canceled almost a third of flights from Britain's busiest airport Sunday, plunging travelers into a fourth day of travel chaos triggered by the foiling of an alleged airplane bomb plot.

British Airways said it had scrapped 30 percent of flights from London's Heathrow Airport on the instructions of the airport operator British Airports Authority, or BAA.

The airport operator said 70 percent of flights on all airlines at Heathrow were expected to operate, but warned that tough new screening measures for passengers and luggage meant delays were inevitable.

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BAA's chief executive for Heathrow, Tony Douglas, said the airport was doing its best, but that delays "will go on until the security threat level is reduced."

Home Secretary John Reid acknowledged that the security measures would have to be altered.

"The present regime is time limited," Reid told British Broadcasting Corp. television. "We know it is not indefinitely sustainable."

After the foiled plot, authorities introduced tough new security measures, including individual searches of all passengers and a ban on carry-on luggage except clear plastic bags containing travel documents and a few essential items.

Reid said Transport Secretary Douglas Alexander had asked U.S. Homeland Security Secretary Michael Chertoff to ease the restrictions imposed by the United States, which include a ban on passengers carrying liquids aboard flights.

On Sunday, British Airways canceled almost 100 flights to Europe from Heathrow and scrapped all its domestic flights from London's second airport, Gatwick. Most long-haul flights were operating, although 10 BA services to the United States were canceled.

Scores of flights from Britain to Europe and the United States also were canceled Saturday. Passengers were delayed so long by the strict new security measures that many missed their flights.

British Airways said in a statement that it "cannot rule out the possibility that flights will once again depart without all their passengers because of ongoing problems with BAA's security search process and baggage operation at Heathrow Airport."

Some airlines accused BAA — which operates seven of the country's major airports — of failing to cope with tough new anti-terror security requirements brought in after police said they had foiled a plot to blow up as many as 10 trans-Atlantic jets in mid-air.

"The airport's baggage system cannot process all of the passengers' bags and where passengers have been able to check their bags in, the lengthy queues in the airport security search area means that passengers are unable to get to the departure gate in time for their flight," BA Chief Executive Willie Walsh said.

Budget carrier Ryanair appealed to the British government to use police and army reservists to speed up searches at overloaded airport security checkpoints, and said the requirement to search all passengers should be eased.

Ryanair, which has its British base at Stansted Airport, northeast of London, said it had complied with BAA orders to cancel more than 60 of its Stansted flights this weekend, about 20 percent of the total.

"We don't need to be body searching young children traveling with their parents on holiday to Spain," chief executive Michael O'Leary told Sky News television.

"If we the industry and the government don't work together to have sensible security ... we are going to hand these extremists a terrific PR success," he added.