Archbishop is backing Heathrow demo and risking wrath of PM

The Archbishop of Canterbury is backing a mass demonstration against a third runway at Heathrow, putting him on a collision course with Gordon Brown.

Dr Rowan Williams will risk the wrath of Downing Street by supporting the march on Heathrow this month. Tens of thousands of campaigners are expected to protest about expansion plans.

Dr Williams, who has almost given up taking flights, will write a letter of support to be read out at the march. He will not attend because of other commitments.

Mr Brown has said that the expansion of Heathrow is "desirable". An announcement on a third runway, sixth terminal and a huge increase in flights is expected soon.

The decision by Dr Williams effectively to come out against the expansion plan will give a huge boost to the campaigners - a broad alliance of environmentalists, councils and local residents.

The Archbishop's spokesman told the Evening Standard: "He is writing a message of support. He acknowledges the strength of support (against a third runway). He is aware of the problems encountered by churches on the ground across London. He is trying not to fly if he can help it. He has not flown at all this year."

The Archbishop is currently in Rome where he is visiting the Pope and will travel back to London by train.

It is not clear if his letter will spell out his direct opposition to the Heathrow expansion plan - or merely warn of the general dangers of climate change. The organisers of the Make A Noise Carnival on 31 May will declare the support of Dr Williams in an official announcement today.

The event's spokeswoman Tamsin Omond, who was arrested this year for staging a rooftop protest at Parliament, said: "It is great to have the Archbishop supporting this march against a third runway at Heathrow.

"While he has made statements in the past warning of the dangers of climate change, it is wonderful that he has shown willing to support a specific demonstration against Heathrow.

"It definitely seems to put him on a collision course with Gordon Brown.

We have not been led to believe anything other than the Archbishop is against the third runway. It is very impressive."

The protest will begin with a march from Hatton Cross Tube station, through the east of the airport along a private BAA road and ending at Sipson, a village which will be largely demolished if the expansion goes ahead.

Downing Street refused to be drawn on the Archbishop's support. "It is a matter for him," said a spokesman, adding: "In his speech to the Institute of Directors on 30 April, the Prime Minister reiterated that subject to all the various criteria, Heathrow expansion was desirable." Transport Secretary Ruth Kelly will make an announcement on BAA's plans following a consultation which ended in February but which has been derided as a fix.

Ms Kelly has said she backs Heathrow's expansion provided strict pollution and noise tests are met.