The backers of an alternative extended runway scheme for Heathrow are preparing for a judicial review of the government’s Airports National Policy Statement.

Legal advisers to the Heathrow Hub consortium have written to transport secretary Chris Grayling highlighting “serious errors” over the policy – which includes a third runway at the London hub – by the Department for Transport.

The group proposing an extended runway at Heathrow rather than building a new one warned that the NPS will “likely be legally challenged” under Section 13 of the Planning Act.

Section 13 allows for a judicial review of an NPS in the six weeks after it has been designated, following a vote in Parliament. NPS due to be voted on by MPs in the coming weeks.

Heathrow Hub argues that the government wrongly claimed in the NPS that an extended runway offers lower capacity of 700,000 movements compared to 740,000 for the third runway.

The issue capacity is one of the principal reasons why the DfT did not select the extended runway and why it has refused Heathrow Hub’s requests to include it in the NPS, according to the consortium.

Heathrow Hub legal advisers DAC Beachcroft has submitted a 21-page report to the DfT “clearly demonstrating” how the department failed to correct a mistake claimed to be made by the Airports Commission which backed a third runway at Heathrow.

A complaint previously made to the Competition and Markets Authority about Heathrow is also ongoing.

Heathrow Hub director Jock Lowe said: “Parliament is due to vote on the NPS in a couple of weeks and MPs should not vote for it.

“If they approve the NPS in its current form, there will be a judicial challenge soon after.

“The errors the DfT have made in relation to capacity are fundamental and explain why passengers and airlines are being saddled with Heathrow airport’s expensive, complicated third runway instead of our cheaper, simpler, quieter plan.

“Chris Grayling should be under no illusion that, not for the first time, his department has bungled the details of a critical transport project. He should withdraw or amend the NPS before it is too late.”