The Conservatives' star candidate said in an interview with LBC: "It’s a pledge of course I'd honour."
Though he added he did not think he would have to trigger a by-election in his constituency:
It’s a nuisance for a lot of people and we’re going to have to fight like hell for the next eight months, but the decision is inevitable: we’ve won all the arguments again and again. Now politics will determine this, and it’s not political deliverable any more than it's legally deliverable.
This is not the first time that Goldsmith has said he will resign if a third runway is built at Heathrow.
David Cameron, meanwhile, would have to renege on his promise made in 2010 if he went ahead with the plans: “No ifs, no buts, there’ll be no third runway at Heathrow.”
Goldsmith makes his comments days before the first Conservative mayoral hustings on Saturday.
Mark Littlewood, director general of the Institute of Economic Affairs, however, said with a greenlight from the Airports Commission, it’s now crucial that politicians stop hampering expansion at the expense of economic growth.
Unless the government steps back from the capacity debate, the recommendations from the Davies Commission will be completely futile. Six years of navel gazing and politicking is more than enough. It’s time runways are built, to ensure that the interests of passengers and the wider UK economy prevail.