The airport has permission to increase the number of flights operating from it but to accommodate them needs to extend its infrastructure, including lengthening runways and building more carparks.
Its owners, and local Newham council which backs the scheme, claim the expansion would add £750m per year to the UK economy and create 2,000 new jobs in Newham and East London.
However campaigners oppose the scheme, arguing it would create more noise and disrupt local residents.
Mr Johnson has now sided with the objectors and ordered the council to refuse planning permission because the airport did not “adequately mitigate and manage” the scheme’s “adverse noise impacts”.
The Mayor’s decision was welcomed by John Stewart, chairman of HACAN East group which campaigned against the expansion, who said: “The airport is paying the price for being so cavalier about noise.
“Quite simply, Boris did not believe its claims that it was dealing adequately with noise. We salute his decision”
The airport said it was “perplexed and disappointed” by the Mayor’s decision.
CEO Declan Collier added: “It is ironic that the Mayor of London, whose platform has always been one of advantage for business in London, is denying the capital the business opportunity presented by growth at London City Airport.”
Sir Robin Wales, Mayor of Newham, accused Mr Johnson of “blatant electioneering.”
He said: “Through a rigorous planning process, Newham Council had secured job opportunities for local people, money for vital local services and reduced environmental impacts on residents from the airport.
“The opportunity to create a world-class gateway to the economic and tech centres that are London’s Royal Docks, Queen Elizabeth Olympic Park and Stratford is now under threat.”
