A mayoral aide is facing calls for him to publish a detailed account of his expenses after other members of the London Assembly published their own.
Tory assembly member Brian Coleman is being urged by other members to release an itemised breakdown of expenses - as they have done.
Mr Coleman once claimed more than £8,000 in taxi fares, more on taxis than the entire assembly put together.
Mr Coleman's spokesman said he did not know why his boss had refused.
'Interests of transparency'
"I know he has decided not to publish his expenses but as to why he has decided that I don't know," said the spokesman.
Last year it was revealed that Mr Coleman claimed £656 in just one day on taxis by leaving the cab waiting for two-and-a-half hours.
Twenty-four of the 25 London Assembly members published itemised breakdowns of their expenses claims online on Monday night.
"In the interests of transparency Cllr Coleman should publish his claims," said John Biggs, Labour's deputy leader on the London Assembly.
"His secrecy must be an embarrassment for (mayor Boris Johnson) who has of course had his own problems with excessive taxi bills".