Heathrow will be next, according to the young green activists who invaded a runway at Stansted this week.

More than 50 Plane Stupid protestors cut through the perimeter fence with bolt-cutters on Monday causing the Essex airport to close for five hours.

Spokeswoman Leila Deen claimed the group was already training residents who live near Heathrow in 'direct action' techniques and warned airport chiefs to expect 'something similar if not bigger' than at Stansted.

"I wouldn't say we would do exactly the same thing at Heathrow but they should be watching what we did at Stansted and expecting something similar if not bigger at Heathrow if they do give the third runway the go ahead," she said today.

Campaigners would not elaborate on exactly what they planned to attempt at the world's busiest international airport.

But the activists who climbed onto a plane and unfurled banners earlier this year prove it is possible to gain access to the tarmac.

Any attack is likely to follow transport secretary Geoff Hoon's verdict on expansion plans, including proposals for a third runway, which is due next month.

Mr Hoon announced last week the decision was being postponed as he rejected calls from MPs to give Parliament the vote.

A spokeswoman for BAA Heathrow condemned the protesters' actions but refused to confirm whether security at the airport would be reviewed in the wake of this week's runway invasion.

"Security at Heathrow is very high and has always been an absolute priority," she added.

BAA Stansted has promised to investigate the security breach, adding 'where there are lessons to learn we will do so'.

Police arrested 57 protesters following the demonstration, 49 of whom have been charged with aggravated trespass and bailed to return to court between now and January.

At one point, campaigners claimed airport staff mounted a snow plough and tried to sweep up the eco-warriors, who had chained themselves to a homemade square of fencing.

However, a BAA spokeswoman said the vehicles had been steered into place to block protesters' access to the runway.