LONDON - ABOUT 250 protesters in fancy-dress gathered for a mass picnic at London's Heathrow airport on Monday, police said, as they sought new tactics in their fight against plans to build a third runway.

Amid heavy security, the demonstrators wore clothes in the style of the early 1900s and some dressed as suffragettes, the activists who campaigned to get women the vote and whose peaceful tactics Monday's protest sought to copy.

The government's decision on whether to expand Heathrow, the world's biggest international passenger air hub, is expected within weeks.

It has been delayed as ministers thrash out the pros and cons of the nine-billion-pound (S$19.38 billion) expansion, which has pitted business groups and airlines against environmental groups and local residents.

More than 40 lawmakers from the ruling Labour party and London mayor Boris Johnson, a member of the main opposition Conservatives, are opposed, arguing there is no economic need for a third runway and it will increase pollution.

But business and trades union leaders believe the project is vital to the British economy, and would create at least 50,000 new jobs.

With a string quartet playing in the background, activists at the Heathrow protest - organised by eco-activist group Climate Rush - chatted, sang and sipped from champagne glasses.

'We are trying to emulate what our ancestors did for us to make a positive change, in a civilised fashion, for future generations,' said campaigner Dan Glass, who was dressed as a chimney sweep.

'We have got to do something now because if we don't, the situation will get so much worse.'

A simultaneous protest attended by about 50 people took place at Manchester airport in northern England. -- AFP