A mechanic was left "screaming and petrified" when he opened the bonnet of a van - to find a 4ft (1.2m) snake inside.
The mechanic at the Test Centre, in Bedfont, west London, was finishing an MoT when he made the discovery.
The garage said that when the RSPCA was called officers initially thought the mechanics were playing a prank - but the snake was eventually quarantined.
It was found to be a black rat snake - a constrictor that is not venomous or dangerous to humans.
The species is not native to Britain and the driver of the van said he had not recently been abroad.
Ram Gharu, who works at the garage, said: "[The mechanic] had just finished the MOT and went for one final look under the bonnet.
"Then he screamed - there was an massive snake moving around in there, with its head up.
"He was absolutely petrified."
Mr Gharu continued: "The guys at the RSPCA thought it was a wind up - but we were dead serious.
"The owner of the van was shocked."
The snake has been taken into quarantine by staff from nearby Heathrow Airport.
It is believed to be an escaped pet.