An American Airlines pilot failed a breath test before taking off for an international flight out of London's Heathrow airport, the airline said Thursday.

The Fort Worth, Texas-based airline said the pilot was given the test Wednesday after airport security staff alerted police.

The pilot, whom the airline declined to name, was supposed to operate a flight with 204 passengers to Chicago. The flight was delayed while a replacement was found and the plane eventually took off.

"The company has strict policies on alcohol and substance abuse and holds its employees to the highest standards," the airline's London office said. "Employees at all levels of the company are not allowed to be on duty whilst under the influence of drugs or alcohol and regular screening is carried out."