Concerns over the cost and timescale of Heathrow’s third runway plans are reported to have been raised by the aviation regulator.

The Civil Aviation Authority called on Heathrow to address major doubts over the runway “decisively and urgently” to get the project on track.

Enforcement action against the UK’s biggest airport has been threatened to force it to provide clear evidence about how it would finance the £14 billion runway while avoiding pushing up costs for airlines and passengers, The Times reported.

The regulator also revealed that the project had been hit by a further delay, with a public consultation on detailed plans for the new runway now scheduled for June rather than in the first three months of next year.

In a letter to the Department for Transport, the CAA said that Heathrow must “provide assurance that its revised timetable is realistic” and would “ensure timely delivery” of the expansion.

The letter from CAA chief executive Richard Moriarty, seen by the newspaper, raised concerns over the “lack of high quality and comprehensive information” about how it would keep costs down.

It said that those fears had “not been adequately addressed, despite repeated requests”.

Heathrow must “urgently” provide full details of costs to ensure that it can deliver the transport secretary’s “ambition to keep airport charges close to current levels while being commercially financeable”, it said.

The CAA was considering making a formal demand under the Civil Aviation Act 2012 for more detailed costings.

“We will consider further regulatory action if [Heathrow] fails to delivery appropriate information,” Moriarty is reported to have said.

He added that Heathrow “must now demonstrate to stakeholders that its new timetable is realistic, achievable and allows sufficient time for high-quality engagement while ensuring timely delivery”.

Legislation to pave the way for the project was passed overwhelmingly by MPs last summer.

Heathrow cut £2.5 billion off the costs by pledging to build a smaller terminal and phase the project over several years. It insisted that the move would allow it to keep landing charges close to present levels, a key demand of transport secretary Chris Grayling.

Last month the High Court gave permission for five legal challenges to be brought against the government decision, with a hearing to take place in March.

It is likely that more challenges will be brought when Heathrow publishes its planning application.

Airlines such as British Airways and Virgin Atlantic have repeatedly criticised the project, saying that it is too expensive. Heathrow is already the most expensive airport in the world, with fees adding more than £20 to each ticket, and it is feared that its owners will seek to recoup building costs through even higher airline charges.

A Heathrow spokesman said: “We continue to engage with all stakeholders on our expansion plan and look forward to presenting a detailed preferred masterplan for further public consultation next year. We remain on track to submit a planning application in 2020 and the new runway to open in 2026.”