U.S. airlines canceled UK flights Thursday, waived penalties for ticket changes and extra checked baggage, and asked passengers to arrive earlier in response to tighter security measures after British police said they foiled a plan to blow up trans-Atlantic airliners.

American Airlines, a unit of AMR Corp. (AMR), said it had canceled six flights between London's Heathrow airport and New York, Chicago and Boston. UAL Corp.'s United Airlines (UAUA) said it was expecting delays of more than two hours on flights from London.

Delta Air Lines Inc., which flies to London's Gatwick airport, said its flights were operating normally.

The British government banned carry-on baggage on flights from the UK to the United States.

The U.S. Department of Homeland Security tightened security on domestic flights, prohibiting passengers from bringing any liquids, gels or lotions in their carry-on baggage. It made exceptions for baby formula and medicines.

Delta said it was waiving excess baggage charges, while JetBlue Airways Corp. (JBLU) said it is allowing passengers to check in a third bag without extra fees, as passengers move newly banned items into their checked baggage.

United said it revised its ticketing policy, allowing some people traveling between the UK and United States to make one change to their itinerary without any fees.

Airlines also asked passengers to arrive two hours early for domestic flights.

Stocks of major U.S. carrier were off in the morning, with the Amex airline index down 2.3 percent.