The London Assembly says any “u-turn” over Heathrow expansion would be unacceptable and has urged all London MPs to back their stance.
Expansion at the airport was ruled out by the UK coalition Government but recent reports suggest Ministers could water down or scrap their opposition.
Local campaigners and residents are opposed to expansion, saying it would increase noise pollution and lead to the destruction of homes.
Ministers are due to publish their aviation policy paper shortly and to consult on how to keep the UK’s air capacity in line with predicted need.
Many in the airline industry favour further expansion at Heathrow however Mayor of London Boris Johnson has repeatedly vowed to oppose any further increase in the airport’s capacity.
The Mayor has championed the construction of a new airport in the Thames Estuary, although this is opposed by environmental campaigners and local councillors.
On Wednesday Assembly Members unanimously agreed a motion opposing the construction of a third runway, adding to the pressure on Ministers not to change their minds.
Assembly Chair Jennette Arnold will write to all London MPs with the Mayor calling on them “to commit to not supporting any plans for an additional runway at Heathrow, either now or after the next General Election.”
Liberal Democrat AM Caroline Pidgeon, who proposed the motion, said: “However much money the airline industry spends lobbying government, London’s politicians must make it clear to ministers that a u-turn on runways is not an acceptable manoeuvre.”
Gareth Bacon, a Conservative AM, added: “It is not enough for London’s politicians to say no to Heathrow expansion, we also have to work constructively with central government to find a solution to the international transport issues facing the capital.”
The full text of the motion reads as follows:
“This Assembly views with concern recent reports that the Government may be looking again at proposals to bring forward plans to create a third runway at Heathrow, and that there is the possibility that plans for a 3rd runaway may appear in some Parties’ manifestos at the next General Election.
“This Assembly therefore agrees to support councils, community groups and local residents in opposing any plans for a third runway at Heathrow and calls on the Mayor and Chair of the London Assembly to jointly write to all 73 Members of Parliament in Greater London asking them to commit to not supporting any plans for an additional runway at Heathrow, either now or after the next General Election.
“The Assembly further calls upon the mayor and the Government to work together to find alternative solutions to address airport capacity issues in London and the South East””