HUNDREDS of Qantas passengers had to spend the night in a hotel after cabin crew found a snake on board a jet being prepared for take-off.

The tiny serpent was found at the entrance to flight QF21 from Sydney to Narita International Airport in Japan.

"The snake was around eight inches long," a Qantas spokeswoman said. "That's about the size of a ball-point pen."

"It was taken to quarantine in order to determine where it came from."

A replacement flight was organised to Narita and Qantas are working to establish where the unwanted passenger boarded from but it is not thought it was in the luggage compartment.

Passengers were forced to spend the night in a hotel after the incident.

A cabin crew member discovered the snake moments before the 370 passengers boarded the plane.

The re-scheduled flight took off at 7.15am this morning.