* Activists to use toy drones from Sept. 13
* Drone disruption could add to September travel chaos
* Heathrow says illegal acts are counterproductive
* Europe's biggest airport says it can mitigate impact
* Sightings of drones have disrupted Heathrow and Gatwick (Updates with police response in paragraphs 9-10)
By Alistair Smout, Guy Faulconbridge and David Milliken
LONDON, Aug 29 (Reuters) - British climate change activists said they would disrupt London's Heathrow airport with toy drones from Sept. 13, a step they hope will ground flights and put pressure on the government to take tougher steps to reduce carbon emissions.
Heathrow could mitigate the impact of the action, but flying drones would add to travel chaos at Europe's biggest airport in September, with strike action by British Airways pilots also planned.
The Heathrow Pause group said it would fly toy drones within a 5 km (3.1 mile) restricted zone around the airport but outside the flight paths of the airport, a step the group said would force the airport to ground flights.
"This is a symbolic action, using a legal loophole and participants’ self-sacrifice to draw attention to the most serious and urgent crisis humanity has ever faced," the group said.
"The government’s inaction on climate change, and the looming catastrophe of airport expansion, gives us no choice and compels us to act."
Heathrow Pause, a splinter group of the climate activism group Extinction Rebellion which has disrupted London with high profile action this year, said it would fly drones at no higher than head level and give the airport one hour's advance notice.
The airport said the plan was illegal and counterproductive but said that it had robust plans in place to make sure the airport could continue to operate.
"We agree with the need to act on climate change. This is a global issue that requires constructive engagement and action. Committing criminal offences and disrupting passengers is counterproductive," a spokesman for Heathrow said.
London's police force said it would do everything in its power to stop drones being flown near the airport.
"Endangering the safety of an aircraft is a very serious offence that can result in a long jail sentence," Deputy Assistant Commissioner Laurence Taylor said.
Heathrow Pause said: "All participants flying drones know they risk arrest and imprisonment, and are prepared to be arrested peacefully."
Drone sightings caused chaos last December at Gatwick, Britain's second busiest airport after Heathrow, disrupting the travel plans of tens of thousands of people in the run-up to Christmas.
The incident led to about 1,000 flight cancellations and affected the travel of 140,000 passengers.
Another drone sighting halted flights for about an hour at Heathrow in January. Both airports have ordered military-grade anti-drone defences.
An Extinction Rebellion plan to disrupt Heathrow with drones during the peak summer season was shelved.
Heathrow had 80 million passengers in 2018, and is set to get bigger, with a third runway approved by lawmakers last year. The expansion faces legal challenges from environmental groups.
The airport already faces disruption next month, with British Airways pilots set to strike on Sept. 9-10 and Sept. 27. Britain's aviation regulator has asked the airline to explain how it has handled the rebooking of customers after complaints.
Britain's BALPA pilots union said it wanted to meet with BA CEO Alex Cruz after he said he wanted a resolution but had not had a response. The union said formal talks were pointless until they heard from Cruz, but BA welcomed the development. (Editing by Janet Lawrence and Gareth Jones)