The Town and Country Planning (Mayor of London) Order 2008 sets out which planning applications must be referred to the Mayor, the process for considering them and the test the Mayor must apply to decide whether his intervention would be justified.
GOL Circular 1/2008 is the Secretary of State for Communities and Local Government's guidance on the arrangements for strategic planning in London. Section 5 of the circular explains how the Mayor's powers over planning applications will operate.
The Mayor's new powers over planning applications came into force on 6 April 2008. Both the Order and Circular are available on the Government Office for London website (link below):