Boris pledges to help fight Heathrow third runway

London Mayor Boris Johnson has waded into the escalating row over the proposed third runway at Heathrow, pledging to make a contribution of £15,000 towards a legal challenge if the government grants approval to the controversial expansion plans.

Transport secretary Geoff Hoon is widely expected to give the go ahead to the plans next month, despite a threatened backbench rebellion on the topic, and widespread criticism that expansion of the airport will undermine the UK's climate change strategy.

However, it now seems that any decision in favour of expansion will be subject to a legal challenge that is expected to claim that a third runway will lead to breaches in EU laws governing air and noise pollution.

A spokesman for the Mayor said his office would provide financial support to the 2M coalition of 21 local authorities, which has been campaigning against Heathrow expansion and is raising funds for a possible legal challenge.

"The Mayor has made it clear that he does not believe the business case for the expansion of Heathrow outweighs the environment concern, and has agreed to provide funding for any legal challenge," he said.

The government has said that it could seek to postpone compliance with EU pollution rules for a short period, but insists that it will not grant approval for a third runway if it leads to an indefinite breach of the rules.

However, critics of the plans, including government watchdog the Environment Agency, have argued that projections on the adoption of cleaner and quieter aircraft are overly optimistic, and have warned that EU limits will be breached.

The announcement by Johnson is the latest in a series of moves by the Conservative Party, signalling its oppositon to expansion of Heathrow.

The Mayor has said that he favours the development of a new airport in the Thames Estuary and recently launched a feasibility study on the proposals, while Conservative leader David Cameron announced last month that a Tory government would scrap plans for a third runway in favour of a national high-speed rail network.