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London mayor Ken Livingstone intends to spend £67,000 at the public inquiry into the proposed 37-storey Heron Tower in the City to defend it from critics opposed to tall buildings.

The figure, which was revealed in a letter from Anthony Mayer, chief executive of the Greater London Assembly, to Tony Arbour, chair of the GLA's planning committee, marks Livingstone's determination to push though his tall building policy. A successful outcome at the hearing, which began this week, is crucial to Livingstone's aim of presiding over the construction of 20 towers in the next 10 years.

The bulk of the £67,000 will be spent on fees for counsel, but £7000 has been used to secure the services of Will Alsop as an expert architectural witness. Lord Rogers will also give evidence on behalf of Livingstone in his capacity as adviser to the mayor.

The mayor's main opponent is English Heritage, which claims, among other things, that if the tower is built it will obscure views of St Paul's Cathedral.

Livingstone's decision to set aside this amount of money to argue his case has been criticised as wasteful by members of the Greater London Assembly. Arbour said there was no need for Livingstone's team to have spent public money on giving evidence orally. He said: "Livingstone could have simply written to the inspector and asked for his views to have been presented as evidence."

Arbour believes that Livingstone's team will repeat evidence given by other supporters of Heron Tower, such as the Commission for Architecture and the Built Environment and the building's developer, Heron Properties.

The bulk of the £67,000 will be spent on legal fees, but £7000 has been used to secure Will Alsop as an expert witness

But Mayer said in his letter that Livingstone had the right to be represented at the inquiry. He said: "It is for the mayor to decide how his views should be represented, and by whom … it was felt that the importance of this appeal more than justified having witnesses in attendance."

Mayer added that most of the other parties were likely to spend more than £67,000.

The controversy about costs deepens the rift between the mayor and assembly members over Heron Tower. The assembly voted unanimously in favour of opposing the scheme in February, resolving that the mayor should have consulted the assembly planning committee before making his decision to support the proposal.

Arbour has since written to the planning inquiry inspector asking that the views of assembly members should be included in the evidence against the scheme.

The tower, which was designed by US architect Kohn Pedersen Fox, would be built at Bishopsgate in the City of London. The inquiry into it is expected to last six weeks.

Ken Livingstone is asking for an inquiry into developer Minerva's proposal to build a £500m shopping mall scheme in Croydon. Livingstone made his request in a letter to planning minister Lord Falconer, the second time he has requested a public inquiry into the project.