Sadiq Khan has been placed under investigation for accepting free tickets to a Taylor Swift concert last summer.
On Friday, City Hall's standards watchdog confirmed it would be examining whether Labour's Mayor of London "exercised an appropriate level of caution in deciding to accept the tickets" to the show last August.
Khan, who was handed a knighthood for "public service" in the New Year Honours, initially said he had received six tickets worth £194 each from the Football Association, the BBC reported.
But officials later back-tracked, saying the tickets were actually worth around £500 each - and had been paid for by a company called LS Events, thought to have worked with City Hall in the past.
Khan's office has said "an administrative error which was corrected" was behind the U-turn.
Sadiq Khan has been placed under investigation for accepting free tickets to a Taylor Swift concert
PA
The investigation comes after Conservative London Assembly member Susan Hall lodged a formal complaint against long-time opponent Khan - though three of her four allegations were dropped by authorities.
A Greater London Authority (GLA) spokesman said: "The monitoring officer has reviewed the complaint against the Mayor and will not be investigating three of the four allegations.
"An investigation will now take place to establish if the Mayor exercised an appropriate level of caution in deciding to accept the tickets."
Khan initially said he had received six tickets worth £194 each from the Football Association - before admitting that their true cost fell at around £500
PA
Meanwhile, a spokesman for the Mayor of London said: "Any gift accepted by the Mayor is declared openly and transparently. In this case, there was an administrative error which was corrected.
"The Mayor has no involvement in the procurement process for GLA events, nor in the tendering of these contracts.
"The Mayor's office will continue to ensure all the right processes are followed, and looks forward to explaining the approach that was taken in this instance."