Boris Johnson is under new pressure to reveal details of the 'secret' deal to rent London's Olympic Stadium to West Ham.
The London Assembly is demanding to know how much money will be repaid to taxpayers if the club is sold.
A report by the Assembly's Regeneration Committee suggests West Ham could be worth £400m after moving to the Olympic Park next year.
Taxpayers face a bill for £190m to covert the £429m athletics stadium into a football ground.
West Ham's deal is shrouded in secrecy but the club is said to have contributed £15m to the conversion costs on top of a reported annual rent of £2.5m.
The Assembly has called on the mayor to disclose details of a so-called 'clawback' if the club's owners sell.