The protest against Heathrow's third runway expansion is expected to be attended by thousands

Thousands of people are expected to attend a protest opposite the Houses of Parliament against proposed plans to build a third runway at London's Heathrow airport. The rally in Parliament Square on 10 October is expected to be the "biggest ever" demonstration against the plans to expand the airport in west London. All five candidates in the upcoming mayor of London election, including Conservative candidate Zac Goldsmith and Labour candidate Saqid Khan, are expected to speak at the event.

Hundreds of residents in west London who will be affected if the expansion gets the go-ahead will also attend. John Stewart, chair of Heathrow Association for the Control of Aircraft Noise (HACAN), the campaign group that is one of the main organisers of the 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 at Heathrow. 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."

The Airports Commission previously backed a third Heathrow runway, claiming it would give the economy a £147bn ($225bn, €198bn) boost, and produce 70,000 jobs by 2050. The report's author, Sir Howard Davies, said recommendations for the location of the new runway were "clear and unanimous".

The protest in Westminster is taking place a few weeks before David Cameron is due to make a decision on whether Heathrow or Gatwick will get the expansion, with the government reported to be following the Commission's recommendations.

Goldsmith, who helped organise the rally, said the number of people expected to attend will present a "united response" against the commission's findings. He added: "The environmental case against a third runway is devastating, and makes expansion both legally and morally impossible. The economic case has completely disintegrated. I am confident that the government will make the right decision."

A protest sign is displayed in the village of Sipson that would be demolished should a third Heathrow runway be built Getty

The report was also attacked by Mayor of London Boris Johnson, who argued the document showed expanding the airport would offer six fewer long haul destinations a day and the number of British destinations reached from Heathrow will be reduced from seven to four by 2030.

He said: "The Airports Commission has spent several years in the production of a gigantic ball of wool that they are now attempting to pull over the eyes of the nation. Their report very clearly shows that a third runway will fail both London and the UK on every level. Our great nation is sleepwalking its way towards becoming a bit part player in the aviation world."

Johson's comments were rejected by John Holland-Kaye, chief executive of Heathrow, who argued the expansion at the airport will "more than double" the number of domestic routes it serves to at least 15. He told The Independent: "As the UK's only hub, we take seriously our responsibility to promote growth across the country.

"That's why we recently announced a new package of commitments designed to safeguard and improve domestic connectivity at Heathrow. We have already started talks with airlines which could more than double the number of domestic routes into an expanded Heathrow."