The accident occurred in December last year, according to The Art Newspaper, but has only just come to light.
The waiter, who was from an external company, had bent down underneath the Townley Venus and knocked its right hand while getting up again.
The thumb was knocked clean off the statue and fell to the floor intact. The statue has now been restored.
The British Museum said it had taken the incident "seriously", the BBC reported today.
"This was an unfortunate incident. The preservation of the collection is of fundamental importance," a spokesman for the museum said in a statement.
Also Read
The Townley Venus is already missing its index finger, which was broken before the sculpture came to the British Museum.
The sculpture, which depicts a half-naked figured of the goddess of love, is a marble copy of the Greek original and dates from the 1st or 2nd Centuries.
It was excavated in 1775 from the baths at the port of Ostia in Rome and bought by English collector Charles Townley.
It was sold to the British Museum in 1805. The Greek original dates back to the 4th Century BC.
Disclaimer: No Business Standard Journalist was involved in creation of this content