The new prime minister is being urged to select a cheaper option for expanding Heathrow rather than the airport’s £14 billion third runway plan.

The call came from the backer of alternative runway scheme Heathrow Hub as the Civil Aviation Authority published a new consultation on expansion of the UK’s largest airport.

While the regulator believes the overall costs are reasonable for such a huge infrastructure project, it raised concerns over the airport’s forecast of “significantly increased” £2.9 billion costs before receiving final planned consent for the third runway.

Heathrow has said it needs to bring forward the timing of certain spending before the expected late 2021 date of receiving the green light for development, with the new runway projected to open by 2026.

“In the light of these increases, we have asked HAL [Heathrow Airport Limited] to consider different options for this spending and the implications of tis spending for the overall programme timetable and the interests of consumers.”

The CAA highlighted the risk of early expenditure being passed onto consumers through airport charges in the document which is out for consultation until August 22.

HAL currently estimates that total expansion costs are now around £32.4 billion in the period to 2050 to provide facilities to handle 142 million passengers a year.

“As well as HAL’s work to develop its capacity expansion programme it is important to note that other important changes will be required to allow airlines and passengers to take advantage of new capacity at Heathrow airport,” The CAA said.

“In particular, airspace modernisation will be required across the south-east of England to allow for aircraft to make best use of airspace and reduce the impact of increases in air traffic on the environment.”

In reaction, a Heathrow Hub spokesman said: “At last the CAA is starting to focus properly on the huge costs of Heathrow airport’s own plan.

“It is an outrageously expensive and disruptive proposal, which involves knocking down 780 houses, BA’s headquarters and the Lakeside Energy from Waste Plant.

“Their plan also involves trying to build the new runway over the M25 right by the M4 junction and will cause years of jams.

“As the CAA concedes, this will potentially have an important impact on consumers via higher air passenger fees.

“Instead of prevaricating, Boris Johnson and Jeremy Hunt should admit the third runway is in big trouble and go for our cheaper, simpler, greener option instead.”

A CAA spokesperson said: “Our latest consultation is clear that, in the view of third party advisors, Heathrow Airport Limited’s current cost estimates are reasonable.

“We welcome the work Heathrow has done to bring the forecast costs down. But we also recognise that the more Heathrow’s operator spends before it gets planning approval, the greater the risk that consumers will end up paying for extra costs in the event that expansion does not go ahead.

“We have asked Heathrow Airport Limited to look at all options to see how this risk can be reduced and to engage with airlines on the best approach.”