John McDonnell adds support to letter sent to Crown Prosecution Service, while all five mayoral candidates plan third runway protest outside Parliament

The shadow chancellor John McDonnell, lawyers and environmentalists campaigning to prevent a new runway at Heathrow have written to the Crown Prosecution Service (CPS), asking them to drop charges of aggravated trespass against 13 activists for a protest at the airport.

The actions of the protesters, which prevented a number of planes taking off, were “reasonable, justifiable and honourable”, according to the letter. “We should be congratulating them for defending the planet, not prosecuting them,” it says.

The letter was sent on Friday, the day before a protest outside Parliament at which the London mayoral candidates from all five main political parties, including Zac Goldsmith and Sadiq Khan, will speak against the building of the third runway.

The Airports Commission set up by the government selected Heathrow for runway expansion near London on 1 July, saying it was the “clear and unanimous” choice over options at Gatwick and elsewhere.

But there has been strong opposition from local groups and politicians concerned about air and noise pollution and from climate change groups.

Following a protest at Heathrow on 13 July, 13 people were charged with aggravated trespass and being in a restricted area of the airport without permission. All 13 have pleaded not guilty and the case is expected to be heard in January.

Heathrow airport is in the Hayes constituency represented by McDonnell, who is a steadfast opponent of a new runway and will also speak at Saturday’s protest. He said last month that he could remain shadow chancellor even if Labour back the Heathrow runway, as it has previously. “We respect the individual constituency MP’s view to protect his constituency,” McDonnell said.

The letter to the CPS, calling for the charges to be dropped “in the public interest”, includes human rights lawyer Michael Mansfield QC and Green Party MP Caroline Lucas among a dozen signatories. It says: “Successive governments, including the present one, have failed to act appropriately [in cutting carbon emissions]. A new runway, and the many thousands of extra flights it would allow, would make the necessary cuts far more difficult to achieve.”

A spokesman for Heathrow said: “Heathrow supports the right to peaceful protest within the law, but the safety and security of our passengers, aircraft and colleagues, together with the smooth running of the operation is paramount. We have been clear that anyone who breaks the law can expect to face legal action.”

Facebook Twitter Pinterest An anti-Heathrow airport expansion protest sign in the threatened village of Sipson, adjacent to the airport. Photograph: Peter Nicholls/Reuters

David Cameron has said the government will make a decision on the Heathrow runway before the end of 2015.

In 2009, before becoming prime minister, he said: “The third runway at Heathrow is not going ahead, no ifs, no buts.” Like Labour, the Conservatives are having to deal with high-level opposition to a new runway. Foreign secretary, Philip Hammond, and international development secretary, Justine Greening, are firmly against a new Heathrow runway: both represent constituencies under Heathrow’s flight path.

John Stewart, chair of anti-runway campaign group Hacan, which is a key organiser of Saturday’s rally, said: “Thousands are expected to send a clear message to the prime minister that they will fight any decision to give the green light to a new runway. And the message from all the key mayoral candidates is equally clear. They stand united on this issue. They are all firmly against a third runway.”

Greenpeace UK executive director John Sauven, who will also speak on Saturday and signed the CPS letter, said: “A double whammy [of air pollution and carbon emissions] isn’t a price worth paying to allow a minority of wealthy frequent flyers to fly even more, at the expense of the majority of us.” He said international business travel was declining and that aviation emissions would take up at least 25% of the UK’s entire emissions in 2050 under government plans.

The Heathrow spokesperson said: “Independent analysis by the Airports Commission found building and operating a third runway at Heathrow is compatible with the UK meeting its long-term climate change reduction targets. The independent Committee on Climate Change has also shown that a 60% growth in passenger numbers can be achieved within climate change targets.”

The other signatories of the letter to the CPS are: Natalie Bennett, Craig Bennett, Nnimmo Bassey, Bill McKibben, Suzanne Jeffrey, George Monbiot, Damien Short, as well as Stewart and Sauven.