CHANCELLOR Rachel Reeves’s Heathrow claims were based on a report commissioned by the airport itself, it has emerged.

Ms Reeves has said a new third runway would create 100,000 new jobs and that: “According to the most recent study from Frontier Economics, a third runway could increase potential GDP by 0.43 per cent by 2050.”

The Frontier Economics consultancy has confirmed that it had been commissioned by Heathrow to write the report, but insisted it had been an entirely independent exercise.

Heathrow has published only the report’s ­executive ­summary online.

Today socialist MP John McDonnell, whose constituency includes Heathrow, said: “Within five days of Chancellor’s announcement of her backing a third runway at Heathrow every thread of her argument is being pulled apart.

“In Parliament I asked twice for publication of the papers she based her decision on. Now we know why we haven’t seen them.”

Alex Chapman, senior economist at The New Economics Foundation (NEF) think tank, said: “Heathrow expansion represents a major threat to the UK’s climate goals and flies in the face of scientific advice.

“It is very concerning that the chancellor appears to be basing her support for Heathrow expansion on a figure from a report commissioned by Heathrow airport.

“Even more worrying is the fact that the methodology they have applied is one that the Department for Transport has previously decided is not fit for purpose, and that the report uses forecast data supplied by the airport itself.”

NEF analysis has also refuted the economic case for expanding the airport, “not least, the decline of business air travel, the surge in outbound leisure travel and the negative impacts on wider regions of the UK — all of which erode the potential growth benefit,” he said.

The Treasury was approached for comment.