Tory MPs have threatened to reject the party’s manifesto if it promises to expand the major hub, the Sunday Telegraph reported.
A Conservative spokesman told the newspaper there is “no need” to cite the project despite it not being voted through because it was announced in October last year.
The Conservatives pledged not to build a third runway during the 2010 General Election campaign, however the Theresa May has now backed the project.
But the Liberal Democrats have warned the Prime Minister she “won’t fool anyone” if plans for the airport’s expansion are absent from the Tory manifesto.
Sarah Olney, whose south-west London seat lies beneath the airport's flight path, responded to reports that an election pledge has been resisted by Foreign Secretary Boris Johnson and other Conservative critics.
Richmond Park MP Ms Olney said: "Despite their earlier promises, the Conservatives are dead set on building a third runway at Heathrow.
"That would be disastrous for the environment; won't deliver the economic benefits the Government claim; and will be a blight on the lives of residents across south-west London.
"Failing to mention their commitment to expanding Heathrow in their manifesto won't fool anyone.
"It's clear that Conservative candidates in this election are standing on a platform of building a third runway at Heathrow."
The Liberal Democrats have consistently opposed Heathrow expansion, plans for which are currently under consultation.
Ms Olney unseated Zac Goldsmith as Richmond Park MP in December after he triggered a by-election by quitting the Conservative party in protest at its support for a third runway.
Mr Goldsmith was recently selected by the Tory party to stand in the seat again, although he will continue to oppose the plans.