Ken Livingstone's new London Views Management Framework, which comes into force in July, will see protected 'view corridors' narrowed to allow for more development in the capital.
Assembly deputy chair Brian Coleman, said: 'Hundreds of years of history and decades of effective protection could now be gone within months.
'For Ken's developer friends Christmas has certainly come early.'
The Department of Communities and Local Government (DCLG) admitted the views will be narrowed, but claimed the Mayor's plans will mean a better balance for development and the protection of heritage, insisting views will not be obscured.
A DCLG spokesman said: 'Views of St Paul's will still be protected under the new guidance. Views will be narrowed, not obscured, because the Mayor has decided that was the right balance to be made between the need for new buildings in the capital and protecting our heritage.
'There was no national policy reason to disagree with the Mayor's approach, and he is responsible for planning issues across London.'