Mr Johnson said he did not believe the payments broke any rules of conduct
Boris Johnson has said he will pay back taxpayers' money spent on his hotel room at the Tory Party conference if he has broken official rules.
The London mayor spent almost £2,000 on accommodation for himself and a small group of advisers at the Hyatt Regency Hotel in Birmingham last year.
Regulations state the mayor may not use Greater London Authority money for party political purposes.
But Mr Johnson said he did not believe the conference fell into that category.
He said he was there to give a speech in an official capacity and there was "no reason at all" why the bill should not have been paid by the taxpayer.
The Taxpayers' Alliance said the mayor's trips to the conference were clearly in his own interest and that of the Conservative Party, and were not proper use of taxpayers' money.
Matthew Sinclair, research director, said: "If he wants to go to a party conference he should do so in his time and at his own expense."
He said the spending of money on the hotel room in Birmingham "hurts the confidence" of taxpayers in London.
But Mr Johnson told BBC One's Andrew Marr Show that the cash had been spent on rooms for himself and several members of his team, pointing out that it was just one of a long list of events that he had to attend.
He was not present at either the Labour or Liberal Democrat conferences.
"The mayor goes to party conferences and there is no reason at all why his bill should not be paid by the taxpayer," Mr Johnson said.
"I don't believe is it proscribed by our rules of conduct but if it is proscribed then I will certainly pay it back."
He also said details of the spending would not have emerged during the tenure of former mayor Ken Livingstone, hailing a new era of "transparency" at City Hall.
A spokeswoman for the mayor said: "Boris Johnson attended the conference in his capacity as mayor of London.
"He was accompanied by his director of communications and one other aide to handle his schedule of meetings and media.
"The figure referred to includes their costs and those of his deputy mayor for policy and planning, Sir Simon Milton."
She said the mayor drove himself to the conference, at his own expense, and spent one night only in Birmingham in a standard hotel room, not a luxury suite, and his hotel bill was £270.
She added that in addition to a speech, the mayor held meetings with borough leaders and other "stakeholders", addressed a leading think-tank and gave media interviews.
But John Biggs, Labour's deputy leader on the London Assembly, demanded a "full explanation" from the mayor and his advisors.
"They must understand that City Hall is not their piggy bank," Mr Biggs said.
"All expenses must be properly justified and in the interests of London, not a political party. Nobody, however powerful, is exempt from the rules."