Flawed evidence was used by the Airports Commission to back a third runway at Heathrow, rival Gatwick reportedly claims.

The commission used selective analysis to exaggerate its case in favour of Heathrow when it reported eight months ago, the Times reported.

Gatwick accused the commission of “triple counting” some international passengers who pass through Heathrow.

Gatwick believes that the commission overstated the impact of a third runway on passenger numbers and long-haul traffic, after submitting freedom of information requests to the Department for Transport.

It also compared the situation with the west coast mainline tendering fiasco, when the award of a contract to run trains to and from London was scrapped because of errors in the accounting process.

The Airports Commission led by Sir Howard Davies (pictured), now chairman of Royal Bank of Scotland, insisted that a third runway at Heathrow would deliver up to £147 billion of economic benefits to the UK and would serve 75 profitable long-haul destinations every day by 2050, 12 more than a two-runway airport.

Gatwick’s bid for a second runway was largely rejected by the commission, which concluded that its expansion would create only £89 billion of benefits and would serve 21 long-haul routes, only one more than under the existing set-up.

Sir Howard has twice rejected claims from Gatwick that he got his sums wrong, telling ministers that they should ignore the airport’s “entirely misguided” pleas, the newspaper reported.

Meanwhile, London mayor Boris Johnson yesterday released a report using World Health Organisation data to claim that the health impacts of expanding Heathrow could cost up to £25 billion over six decades.

“He said that the best move would be “to build a four-runway hub airport at the Thames estuary or Stansted”.

The mayor, in a report ‘Landing the Right Airport,’ argues that a four-runway airport east of London is the only way to secure enough capacity.

“If we are to secure the connectivity we need to support our future growth and prosperity and do so without dire impacts on public health – then we must do better than Heathrow,” Johnson said.

Building an airport at one of two locations in the Thames Estuary or expanding Stansted in Essex “away from populated areas” was the “only credible solution”.

In his forward to the 78-page document, Johnson added: “Each could accommodate the four-runway hub that London and the UK needs.

“Our analysis predicts that they would offer around double the number of long haul and domestic destinations served by Heathrow today, while exposing 95% fewer people to significant aircraft noise.

“A four-runway hub to the east of London, rather than jarring with the growth of London will support it, catalysing regeneration and housing to the east.”

The government is due to rule on the location of a new runway in southeast England in July after delaying a decision in December.