Boris Johnson makes spectacular planning U-turn

In July the Mayor opposed the scheme in Waterloo, known as Elizabeth House but nicknamed the Three Ugly Sisters, which Lambeth council wanted to approve. This week he withdrew his objections.

The developers still have to face a public inquiry after Communities Secretary Hazel Blears decided to examine the proposals over their impact on historic views and sites.

Her decision leaves the Mayor accused of flip-flopping by groups opposed to the development. His election campaign relied heavily on his commitment to protect historic views but he has been accused of betraying these promises.

In July, Boris rejected the plans by developer P&O for buildings up to 33 storeys high, due to their impact on "the Westminster World Heritage Site; the view from the footbridge in St James's Park, and the setting of County Hall and the Royal Festival Hall". The decision was seen to confirm the Mayor's election commitments to the protection of historic vistas.

But on Monday Mr Johnson wrote to Lambeth council: "Having considered a report on this case, I am content to allow Lambeth council to determine the case itself, and do not wish to direct refusal."

The plans now being considered at an inquiry include a 27-storey office building; a 22-storey office building; and a tower of 274 flats reaching 33 storeys. Only 17 per cent of the flats will be "affordable".

Michael Ball, director of the Waterloo Community Development Group, said: "In July Boris said no because of the damage to historic sites. Since then he has had various schmoozes with the developers behind closed doors and, hey presto, the scheme's OK.

"This is the third time Boris has reneged on his election promises to stop skyscrapers, after failing to make effective interventions in Doon Street and Beetham Tower.

"Our only hope now is that the public inquiry will leave Hazel Blears with no option but to reject these damaging towers".

In the case of Doon Street, a block of private flats, Boris declined to write to Ms Blears expressing objections, and she approved the tower in spite of damage to historic views.

The Mayor's objection letter over Beetham Tower, south of Blackfriars Bridge, was drafted in July but did not reach the public inquiry, which ended earlier this month, until September.

A spokeswoman for English Heritage said: "We believe that [Elizabeth House] would cause significant harm to London's historic environment."

A Westminster council spokesman said: "We object to very tall buildings which will harm the settings of listed buildings and important views." The Mayor declined to comment.