Campaigners opposed to a third runway at Heathrow have bought a parcel of the land earmarked for the airport’s expansion and are preparing for a fierce legal battle to defend it.

The Government is expected to approve the new runway this week, along with a sixth terminal – although there was speculation last night that a decision could be delayed after Gordon Brown agreed to meet Labour backbenchers opposed to the project. The Conservatives and the Liberal Democrats both oppose the plans, as do dozens of environmental groups.

Greenpeace has bought a field the size of a football pitch and plans to invite protesters to dig networks of tunnels across it, similar to those built in the ultimately unsuccessful campaign against the Newbury bypass in 1996. The group also plans to divide the field into thousands of tiny plots, each with a separate owner. BAA, the airport’s owner, would be forced to negotiate with each owner, lengthening the compulsory purchase process.

Greenpeace believes that the longer the expansion is delayed, the more likely it is that the project will be cancelled.

Times Archive, 1946: Heathrow now in use

A big crowd were at the new London airport today to see the first civil aeroplane take off from the 3,000-yard main runway

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Emma Thompson, the actress, Alastair McGowan, the comedian, and Zac Goldsmith, a Conservative adviser on the environment, were among those who signed the deeds to the site last Friday. They each contributed a small, undisclosed sum towards the purchase, but most of the £20,000 cost was met by a secret donor.

Ms Thompson said: “I don’t understand how any government remotely serious about committing to reversing climate change can even consider these ridiculous plans.We’ll stop this from happening even if we have to move in and plant vegetables.”

John Sauven, Greenpeace’s director, said: “Many thousands of people will be prepared to peacefully defend their field in person, standing in front of bulldozers and blocking construction. This site will become a focus for climate campaigners.”

The Government is planning to attach several conditions to its approval for the runway, in the hope of silencing some critics. It will state that the number of flights at Heathrow will not be allowed to increase if that would result in air pollution and noise levels being breached. Ministers are also considering setting a punctuality target for Heathrow that airlines would have to attain before they were granted additional runway slots.

London First, the business lobby group, called on the Government to produce a mechanism under which the number of take-off and landing slots at the expanded airport would be reduced automatically if punctuality fell below a certain level. Colin Matthews, chief executive of BAA, supported the idea of a target, but refused to say what level he believed it should be.

Heathrow is the worst-performing hub airport in Europe for punctuality, with flights delayed by an average of 16.1 minutes in 2006, compared with 12.1 minutes at Frankfurt, 12 minutes at Amsterdam and 11.9 minutes at Paris.

Mr Matthews said that the new runway and terminal would cost about £9 billion and would open in 2019 or 2020. Asked by The Times about the 2,000 people who would lose their homes in the village of Sipson to make way for the expansion, he said: “An airport brings good things and bad things. We can’t avoid the bad things. No one can possibly take any pleasure in the destruction that construction can cause.”

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