Photograph: Peter MacdiarmidGetty Pictures The foreign secretary, Philip Hammond, and the London mayor, Boris Johnson, will refuse to support their own partys policy on airport expansion at the subsequent election, potentially opening a rift at the top of the Conservative party. Ahead of a final round of public consultations on the best way to meet demand for additional capability, which will start within days, each Hammond & Johnson have told constituents they’ll strongly oppose the building of a 3rd runway at Heathrow one of 3 alternatives being considered by an independent commission chaired by Sir Howard Davies. The mayor & foreign secretary are amongst a batch of Tories of cabinet or equivalent rank who are expected to rebel against the official party line, which is that no decision on whether to increase Heathrow or Gatwick will be made until Davies, a former director general of the CBI, gives his final verdict subsequent summer. The Observer has been told that Hammond, the MP for Runnymede and Weybridge, made clear the attitude he would take during the election campaign to a third runway at Heathrow when he spoke at a public meeting ten days ago in Egham, in his Surrey constituency.

When asked what his opinion was, the foreign secretary mentioned he was against a 3rd runway & there was a enormous round of applause, said a source who was at the meeting. Those attending voiced their anger about the extra noise & harm to the environment, & concerns about recent trials of new flight paths into Heathrow. Last night Johnson who was lately selected as Tory parliamentary candidate for the west London seat of Uxbridge & West Ruislip also mentioned he would have no qualms about voicing vehement opposition to Heathrow expansion throughout the election campaign, whatever the official Tory party position might be. The mayor told this newspaper: I am wholly opposed to Heathrow expansion, as I have already been throughout my 6 years as mayor.

A 3rd runway would be catastrophic for London and Londoners. Johnson said he remained in favour of a new, four-runway airport in the Thames estuary. Davies has ruled the choice out. The mayor believes his notion, nicknamed Boris Island, may be revived and is the only approach to remedy the capacity concern while avoiding unacceptable environmental effects on Londoners.

The concern threatens to turn out to be a battleground between rival candidates to lead the Conservative party in future. Other leading Tories with south-eastern constituencies who have spoken out against a 3rd runway at Heathrow incorporate the home secretary, Theresa May, who is MP for Maidenhead; the international development secretary, Justine Greening, who is the member for Putney; and the Northern Ireland secretary, Theresa Villiers, MP for Chipping Barnet. The Conservatives went into the last election ruling out expansion of Heathrow, though the prospect was revived in the middle of the parliament after pressure for a rethink from the chancellor, George Osborne. Government insiders and aviation experts now believe that the extent of opposition to the Heathrow choice from politicians of all parties might tilt the benefit in favour of expanding Gatwick, which would have less environmental impact.

The Conservatives and Liberal Democrats each opposed any expansion of Heathrow at the 2010 election. David Cameron ruled it out, saying: No ifs, no buts, therell be no third runway at Heathrow. Ed Miliband voiced opposition when he was secretary of state for climate modify. Even so, in September 2012 ministers set up an independent commission chaired by Davies to assessment the issue.

The commission has already said there’s a need for one supplementary runway in the south-east of England by 2030 & has 3 options: a 3rd runway at Heathrow, lengthening an existing runway at Heathrow, & building a second runway at Gatwick. Heathrow & Gatwick were asked to submit updated proposals to the commission in May of this year. Davies is due to deliver his final report in June. A report commissioned by Gatwick & drawn up by the environmental investigation division of the Civil Aviation Authority will say this week that the number of residents and schools affected negatively by noise as a result of Heathrow expansion would be much further than if Gatwick were developed.

John Stewart, chairman of AirportWatch, an umbrella group for companies fighting unsustainable airport expansion, mentioned the political obstacles to expanding Heathrow were considerable and had become higher since Johnson had been selected for a west London seat.