Heathrow’s boss said a third runway could be just the start of an expansion programme at the airport and the country may need a fourth.
The airport’s new CEO John Holland-Kaye also told the All Party Parliamentary Group (APPG) this week that the hub airport would not decline if expansion did not go ahead.
A Heathrow spokesman said: "We believe that a third runway provides sufficient capacity until at least 2040, allowing the UK to compete with other European hubs.
"It is impossible to accurately predict beyond 2040 as to whether a fourth runway would definitely be required, as long term demand forecasts are inherently uncertain."
The APPG, chaired by Richmond Park MP Zac Goldsmith, is designed scrutinise the arguments for and against Heathrow expansion and includes members who support the third runway, including Lord Soley and Gerald Howarth.
A Heathrow spokesman said failure to expand the airport would spark job losses and the decline of the UK’s aviation hub status.
Mr Holland-Kaye said that if Heathrow did not expand “Britain will not take advantage of the emerging markets that it should be able to”.
Mr Goldsmith said: "We should be in no doubt at all that the debate today isn’t about whether or not there will be a third runway, it is about whether or not Heathrow will have a fourth runway.
"After months of being told that Heathrow will fade away if a third runway is not built, the CEO has admitted that this is not the case.
"In truth, this has always been a straw man argument, put up to frighten people into accepting expansion."
He said Heathrow’s claim an additional 300,000 flights would result in fewer people being disturbed "defies all logic".
He said the Government would be crazy to "cobble together the old monopoly" on one edge of the city at expense to the taxpayer.
Do you support Heathrow expansion? Leave a comment below.