The mayor said the runway would cause “intolerable traffic and fumes in the west of the city – and it will not be built as long as I am mayor of London.”
The Tories ruled out the prospect of a third runway in their 2010 manifesto, but last week chancellor George Osborne used his Budget to acknowledge the need for airport expansion in the South East.
It is understood that a third runway is back on the agenda and will be considered alongside plans for a new airport in the Thames Estuary, which has been backed by Johnson.
Meanwhile, Johnson, who is running for re-election in May, has backed a pledge supporting a referendum on Britain’s membership of the European Union. Johnson added his name to the People’s Pledge on a campaign visit to Romford in Essex over the weekend.
The Mayor said at the end of last year that a referendum would be justified if a new pan-European treaty was in danger of being forced upon the UK.
“I am delighted Boris has signed up,” said Tory MP Andrew Rosindell. “I wish the campaign all the best.”