Scotland Yard said that 51-year-old Erwin Vermont Washington, of Lakewood, Colorado, was arrested after officers were called to United Airlines Flight 949, which was already full of passengers and due to leave London's Heathrow Airport just after noon on Monday.
BAA, Heathrow's operator, said the plane had been due to leave imminently. A BAA spokesman quoted by Britain's Press Association news agency added that the pilot had been reported to authorities by another member of United's staff. BAA did not immediately return a call from the AP seeking comment late Tuesday.
United Airlines spokeswoman Megan McCarthy said the pilot, who she did not identify by name, has been removed from service pending an investigation. She said her airline had strict rules on alcohol "and we have no tolerance for violation of this well-established policy."
She declined to say how long he had worked for the airline.
McCarthy added that the flight was canceled and that the plane's 124 passengers were put on other flights.
Monday's incident bears a strong resemblance to the arrest in May at Heathrow of an American Airlines pilot - also scheduled to fly a plane to Chicago - after he failed a breath test. Airport security staff had alerted airport police about the pilot.
In January, Southwest Airlines put a pilot on leave after passengers at a security checkpoint in Columbus, Ohio, told authorities that he smelled of alcohol. The pilot ran into a restroom and changed out of his uniform jacket and called in sick.
Union leaders say pilots are under increased scrutiny by security agents and passengers because of high-profile cases involving drunk pilots.
Associated Press Airlines Writer David Koenig in Dallas contributed to this report.