DAC Beachcroft, legal advisers to the consortium proposing an extended runway at Heathrow Airport, have written to secretary of state for transport, Chris Grayling, highlighting what it calls serious errors by his department in the Airports National Policy Statement.

The group behind the move, Heathrow Hub, has also indicated it will likely be legally challenged under section 13 of the planning act.

Section 13 allows for a judicial review of a National Policy Statement in the six weeks after it has been designated, following a vote in parliament.

Parliament is expected to vote on planning permission for a third runway at Heathrow in the coming weeks.

However, Heathrow Hub argues its plan for an extended runway is cheaper, simpler, quieter and quicker to construct than Heathrow Airport’s third runway.

The latter proposal is backed by the government.

In the NPS the government has “wrongly” claimed an extended runway offers lower capacity of 700,000 movements compared to 740,000 for the third Runway, argues Heathrow Hub.

The mistake about capacity is one of the principal reasons why the department for transport did not select the extended runway and why it has refused Heathrow Hub’s requests to include it in the NPS.

Jock Lowe, director of Heathrow Hub, said: “Parliament is due to vote on the NPS in a couple of weeks and members 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 department for transport 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.”