Flight BA285 was due to leave Terminal 5 at 10.40am, but the presence of a roaming rodent meant the original aircraft could not be despatched. Because mice have been known to gnaw through wiring on planes, wide-ranging checks must be made to ferret out any problems.

A chewed cable could be located anywhere between the tail and the ram air turbine (RAT).

It took several hours for a replacement 777 to be found, during which British Airways bought lunch for an estimated 200 passengers.

The replacement finally departed from the Cat A airport four hours and 16 minutes late. If it fails to make up time en route, all the passengers will be entitled to €600 (£515) under EU compensation rules. Assuming a similar number of passengers on the corresponding inbound flight from California, BA284, the total compensation bill will amount to £206,000.

In addition, passengers who miss connections either in San Francisco or at Heathrow will potentially need accommodation. The total costs, including engineering, could be within a whisker of £250,000.

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A British Airways spokesperson said: “With service and prices this good, we know almost everyone wants to fly with us to San Francisco, but on this occasion there was one very small customer who we had to send back to the gate.

“Everyone holding their own passport is now on their way to California, and we are sorry for the delay.”