Thirteen activists who chained themselves together on a runway at Heathrow face prison after they were found guilty of aggravated trespass and entering a security restricted area.
The protestors, who are part of the action group Plane Stupid, had staged the protest against Heathrow’s expansion plans.
They cut through a fence at the London airport and invaded the runway, causing 25 flights to be diverted.
District judge Deborah Wright told Willesden Magistrates Court that the cost of the disruption had been ‘absolutely astronomical’.
The seven men and six women will be sentenced on February 24.
Superintendent Andy Jones, of the Metropolitan Police, said: "When protestors encroach into the airside environment they not only cause major disruption but also significant danger to themselves and aircraft.
"These convictions send out a clear message that such behaviour will not be tolerated."
Plane Stupid said in a statement: "When the democratic, legislative processes have failed it takes the actions of ordinary people to change them.
"Climate change and air pollution from Heathrow are killing people now and the government’s response is to spend millions making the problem bigger."
