Until now, the British Museum has not offered any information about the damage, restoration or disciplinary measures, with no clear reference to it in the documents it must publish in the public domain.
The damage occurred on December 10 last year and was reported by The Art Newspaper.
Trustees were informed at a meeting in March, but publicly-available minutes state only that deputy director Jonathan Williams "informed the Board of repairable damage to an object".
The thumb was in fact knocked off during a corporate party at the museum, which rents its gallery spaces out regularly for events.
It is understood that caterers were preparing for the evening in the Ancient Greek and Roman galleries when a member of staff knelt underneath the Townley Venus.
On rising, the caterer's head hit the protruding marble thumb with such force it knocked it off.
The broken thumb was fixed back on with an adhesive in situ, while the gallery was closed to visitors. The work was said to be "straightforward".
The statue, described by The Art Newspaper as "one of the British Museum's most important Roman sculptures", dates from the first or second centuries AD and is a marble copy of a fourth century BC Greek Venus.
Named after collector Charles Townley, it was found in Rome in 1775 and sold to the museum in 1805 when it remains on display.
The arms of the statue were restored in the 18th century, and a finger was also knocked off by a visitor in 2012.
It is understood the catering company were new to the museum, and have not been invited back.
A spokesman said: "This was an unfortunate incident. The preservation of the collection is of fundamental importance.
"Our expert conservators have been able to fully restore the object and it has remained on public display.
"We have taken the incident seriously and have retrained all individuals responsible for events."
She added staff had refreshed their training to be "fully practiced in moving themselves around historical objects" while being "always conscious of the potential risks".
It is the latest in a series of historic accidental damage to sculptures in the museum.
In 2005, the Telegraph revealed nine instances of "minor damage" to the Elgin Marbles from the 1960s to the early 1990, including one incident which saw two schoolboys permanently damage one of the figures when they began fighting in the forecourt
In June 1981, a workman from the Property Services Agency lost his balance and caused part of a glass skylight to fall on the west pediment figure.
In 1966 vandals scratched "four shallow lines" on the back of one of the figures, and in 1970 someone scratched letters on to the upper right thigh of another.
