All the major contenders to succeed Boris Johnson as Mayor of London will speak against a third runway at Heathrow on Saturday opposite the Houses of Parliament.

Conservative Zac Goldsmith, Labour’s Sadiq Khan, Liberal Democrat Caroline Pidgeon, Green Party candidate Sian Berry and UKIP’s Peter Whittle will be joined by campaigners and environmental representatives at the rally, which takes place weeks before David Cameron is expected to make an announcement regarding airport expansion for London and the South East.

The Prime Minister’s cabinet is examining the Davies Commission report, which recommended a third runway at Heathrow in July, as Gatwick chiefs continue to make the case for expansion at the Sussex airport.

Chairman of campaign group Hacan, John Stewart, who helped organise the rally, said thousands are expected to send a clear message to Mr Cameron that any decision to give Heathrow the green light will be fought.

He said: "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."

Mr Goldsmith, who has promised to relinquish his Richmond Park seat should his party grant Heathrow permission to build a third runway, highlighted the fact that the airport already exceeds EU limits for air pollution with its existing two runways.

He said: "An extra runway would add 300,000 extra flights and 25 million more car and lorry journeys a year creating even more pollution.

"Heathrow expansion cannot be reconciled with any of the air quality targets London needs to meet to avoid fines and to prevent early deaths."

Mr Khan conceded London and the South East needed additional airport capacity but said it should not come at the expense of the quality of life of hundreds of thousands of Londoners, so he is backing Gatwick expansion with the possibility of a high speed rail link between the two airports.

The Tooting MP said: "I’ve seen no evidence to suggest it can be done without taking our air quality to potentially illegal levels and blighting the lives of so many people in west London.

"Heathrow needs to be better, not bigger."

A Heathrow spokesman said the airport will work with whoever is elected Mayor next May to help keep London a global aviation hub.

The spokesman said: "Expansion of the airport will connect the UK to the growing markets of the world and secure up to 180,000 jobs and £211bn of economic growth spread across the country.

"Following a three year, £20m study, the Airports Commission unanimously and unambiguously recommended Heathrow expansion. It confirmed that Heathrow can expand within environmental limits."

The spokesman added that its recent polling indicated expansion is supported by more than 50 per cent of residents in the 12 constituencies around the airport, with 33 per cent opposing the plans.