The protesters said yesterday no one had told them directly of the danger. But they said they had stopped using candles in the network to reduce the risk of gas igniting.
Contractors working at the airport first raised the issue. A Manchester airport spokeswoman said they were told there was a risk of natural methane from vegetation at the site and they had passed the information to police.
Chief Inspector Tim Burgess said the gas was one of several risks the protesters face. "We have been warning protesters for the last four weeks about the dangers of tunnelling. These dangers include tunnel collapse, the possibility of methane being present, the lack of oxygen and the standard of the site itself." Police would continue to liaise directly with the protesters, he said.
But Ian, a protester from Wigan, angrily denied methane was a problem or that they had been warned of it. The police had first mentioned the possible danger in an interview on local radio and the protesters suspected it was a scare tactic, he said: "The police didn't come and contact us."
Ian said that safety was paramount and they had experienced diggers carrying out the work, which followed similar tunnel protests at Newbury, Berkshire, and along the site of the A30 protest in Devon.
"We have got geologists who have been in and checked everything and there is absolutely no danger."
About 30 people are on two sites adjacent to the airport at present and more are expected as the year goes on. The demonstrators have set up camp on land bought by Manchester City Council on behalf of Manchester Airport plc for the planned second runway.Reuse content