up, says Conservative MP Zac Goldsmith Conservative MP Zac Goldsmith: "This is about political credibility" David Cameron will face an "enormous bust-up" if he contests the next election promising to expand Heathrow, Tory MP Zac Goldsmith has said. Mr Goldsmith repeated his promise to trigger a by-election in his Richmond Park constituency if his party drops its opposition to building a third runway at the west London airport. A commission has listed this among the UK's options to expand capacity. But Mr Goldsmith said he hoped his party would reject the idea. The three options shortlisted in an interim report by the independent Airports Commission are: adding a third runway at Heathrow lengthening an existing runway at Heathrow building a second runway at Gatwick But Mr Goldsmith told the BBC News the commission's chairman, Sir Howard Davies, seemed to have adopted the view of airport operator BAA, which wants to increase Heathrow's capacity, "hook, line and sinker." Off the fence Prior to the 2010 general election, the Labour government backed building a third runway at Heathrow, but the Conservative manifesto promised to "stop" this. The later coalition agreement reached with the Liberal Democrats, who also oppose expansion, stated: "We will cancel the third runway at Heathrow." Labour has not set out its position since then, and has said it will not rule any runway options in or out while the Davies commission is still deliberating. But the government and opposition have come under increasing pressure to change their minds recently, with airport capacity in south-east England becoming more over-stretched. Sir Howard's final recommendations will be published shortly before the 2015 general election. Mr Goldsmith said the main political parties would have to "get off the fence" when compiling their manifestos, as it was "not possible to enter the next election with ambiguity." He added: "I think there could be an enormous bust-up on this issue. I think David Cameron is aware just how big this could be for him. This is about political credibility... something all politicians want, but pretty much all of them lack. Competitiveness In a statement to Parliament, Conservative Transport Secretary Patrick McLoughlin said: "My principal concern... is to protect the integrity and independence of the commission's process through to the final report, which we expect to be delivered in summer 2015." The government, he added, would not be commenting "on the respective merits of the options which have and have not been shortlisted." For Labour, shadow transport secretary Mary Creagh welcomed the report, and the exclusion of a new runway to the north of Heathrow, but also did not back any of the shortlisted options. She said: "It is vital we take decisions about our airport capacity, including in the south east, which is important for Britain's jobs, growth and competitiveness." Ms Creagh added: "The commission must be allowed to get on and complete its work on the long-term future of aviation." Liberal Democrat Chief Secretary to the Treasury Danny Alexander told BBC News his party was "sticking" to its position of opposing a new runway at Heathrow. He added: "We've always opposed airport expansion in the South East for environmental reasons and it's that environmental test we will apply to [Sir Howard's] final report."