Mayor vows to fight third Heathrow runway Boris Johnson has promised to fight a third runway at Heathrow, amid signs that the government is reconsidering its opposition to the plan to alleviate airport capacity constraints in the south-east. The Conservative London mayor, who is running for re-election on May 3, said a new runway at Heathrow would be "an environmental disaster" leading to "massive congestion" in west London. Launching his transport manifesto, Mr Johnson called for a new hub airport to be built to the east of the capital, arguing this would cut noise pollution and boost the regeneration of east London by providing jobs. The mayor's stance on the third runway threatens to put him at odds with David Cameron, prime minister, who has signalled the need to look again at all the options for relieving the airport capacity crunch. The coalition agreement rules out new runways at Heathrow, Gatwick or Stansted, but the debate has been reignited after Mr Cameron said he was "not blind to the need to increase airport capacity, particularly in the south-east" and senior Tories have called for the policy to be revisited. Mr Johnson said the government might need to include a third runway as one of the options in its consultation as a way of avoiding a possible judicial review, but argued that the plan was "dead" and would not ultimately go ahead. The strongest calls for action on airport capacity have come from business. John Cridland, director-general of the CBI employers" organisation, on Monday said a third runway at Heathrow would be "an obvious medium-term solution." Ken Livingstone, the Labour candidate for the mayoralty, opposes a third runway for the west London hub, arguing short-haul capacity should instead be switched from Heathrow to Gatwick and Stansted to open up more long-haul slots to emerging markets including India and China. A spokesperson for Mr Livingstone said: "Reports that Boris Johnson's Tory government are considering this show the Tory mayor's claims to have successfully lobbied against the third runway are hollow and that people who voted Conservative on this basis have been betrayed." Setting out transport plans, Mr Johnson also pledged that almost half of the rolling stock on London Underground would be automated by 2014, in a move that would "reduce the bargaining power of the union bosses intent on bringing London to a halt." Automation allows trains to run closer together at higher speeds, increasing capacity on the network. The technology, already used on the Victoria, Jubilee and Central lines and the Docklands Light Railway, would be added to the Northern line. However, a staff member would still be present on board for safety reasons. Mr Johnson said the plan to raise efficiency on the network would help him keep fares "as low as I conceivably can," in a riposte to a longstanding pledge by Mr Livingstone to cut fares by 7 per cent in October 2012 and freeze them in 2013.