Boris Johnson was under fire yesterday, after it emerged that he used taxpayers' money to pay for his hotel room at the Conservative Party conference last year. The Mayor of London spent £1,955.25 on accommodation for himself and a small group of advisers at the luxury Hyatt Regency hotel in Birmingham.

Rules governing the Mayor's conduct state that Greater London Authority money must not be used improperly for party political purposes. His office insisted that using public funds was justified because Mr Johnson was attending the conference in September in his capacity as Mayor, giving a speech to delegates.

However, Mr Johnson did not attend the Labour or Liberal Democrat conferences. During his conference address he was cheered loudly when he relished in his triumph over his Labour rival, saying: "Ken Livingstone was terminated!"

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He also stuck to the Tory party line of warning that over-regulation of banking risked driving away jobs.

By contrast, cabinet ministers did not use public funds to attend the Labour Party conference and were, instead, asked to pay out of their own pockets.

Mr Johnson is believed to have arrived in Birmingham on Saturday 27 September, staying for three nights. A suite at the Hyatt Regency, the main conference hotel for senior Tories, would have cost around £1,000 for three nights. The spacious room has a separate office and dining area, a giant bathroom with whirlpool bath and huge windows overlooking the city's canal district. The remaining £955 would have covered the cost of the advisers' bedrooms.

Downstairs in the hotel, Tory activists drank champagne at Bar Pravda – despite a message from the party leadership that they should not be seen to be enjoying themselves at a time of intense difficulty for the economy.

Stephen Pound, Labour MP for Ealing North, said: "If Ken Livingstone had done that he would have been hung, drawn and quartered. Boris has always had a reputation for playing fast and loose with the rules."

On election last May, the Mayor pledged to publish details of all expenditure by him and his office over £1,000. Section 6(b)(ii) of the GLA code of conduct states the Mayor "must, when using or authorising the use by others of the resources of your authority,... ensure that such resources are not used improperly for political purposes (including party political purposes)".

Documents for September show £1,955.25 for "hotels and lodgings". A separate entry lists £2,192.50 for hospitality, but this was spent by a Tory member of the London Assembly.

A spokesman for the Mayor confirmed that the £1,955.25 related to the conference hotel. He added: "This expenditure relates to attendance at party conference. The Mayor attended with a number of key advisers over a period of days. One senior aide stayed for four nights.

"Boris Johnson attended the conference in his capacity as Mayor of London and was a key speaker at the event. Whilst there, the Mayor attended two meetings with London borough leaders and met with countless media, a leading think tank and other stakeholders."