Environmental protesters who occupied a runway at London's Heathrow airport for several hours have been told they face jail after being convicted of trespassing.

Thirteen members of Plane Stupid cut through a fence and blocked the runway in July to oppose plans for a third runway at Europe's busiest airport.

They claimed their actions were necessary to stop people dying from the effects of pollution and climate change.

Judge Deborah Wright said Monday that the protesters were "principled people." But she said the incident was so serious that it is "almost inevitable" they will be sent to prison.

They are due for sentencing Feb. 24.

Heathrow said in a statement that anyone who "interferes with the safe and smooth operation of the airport can expect full prosecution under the law."