The mayor of London has asked the developer of the Walkie-Talkie tower in the City of London schemes to pay £1.4m to the Crossrail transport scheme

Boris Johnson is demanding the money from Land Securities in return for granting planning consent for the controversial scheme, officially called 20 Fenchurch Street.

It is also asking Sellar Properties to pay £1.2m in exchange for passing its Seal House scheme in the City.

But lawyers suggest the mayor is open to legal challenge, as proposed changes to the London Plan requiring developers to cough up of £213.30 per square metre of office floor space have not been approved.

Last month, developer, Terrace Hill was asked for £866,000 towards Crossrail in exchange for planning permission for Howick Square, a 16,000m2 eight-storey mixed-use building.

After negotiation Terrace Hill handed Johnson £120,000 rather than mount a legal challenge, which would have cost about £500,000 and delayed the scheme by six months.