SAN FRANCISCO -- How he squeaked through security is anyone’s guess.

A little mouse made for a big delay on a British Airways flight from London to San Francisco.

The passengers were all buckled up and ready to go when the crew told them that a mouse-spotting meant they couldn’t take off.

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The crew joked that the mouse couldn’t enter US airspace without a passport, and told everyone they needed a whole new plane. That meant a four-hour delay.

An airline official issued a tongue-in-cheek response to CBS San Francisco Bay Area when asked about the delay.

“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,” airline spokesperson Liza Ravenscourt said in an email.