DAC Beachcroft (DACB) has warned that controversial plans for a third runway at Heathrow could face a legal challenge due to "serious errors" by the Department for Transport (DfT).

The firm has written to secretary of state for transport Chris Grayling MP, on behalf of its client Heathrow Hub, to highlight errors in the Airports National Policy Statement, which will be put to MPs for approval in the coming weeks.

Heathrow Hub, which is campaigning for an extended northern runway rather than the new development, says the government "wrongly claimed" that an extended runway would offer lower capacity than a third runway despite being "cheaper, simpler, quieter and quicker" to construct.

DACB has submitted a 21-page report to the DfT outlining the mistake. Planning and environmental partner Christopher Stanwell told Legal Week: "When the airport commission asked people to submit their proposals for Heathrow expansion, it did so on the basis of the proposals expanding the airport's capacity to 700,000 air transport movements.

"It then transpired that it had judged Heathrow Airport Limited's third runway proposal on it increasing its capacity to 740,000 air transport movements. This influenced the Airport Commission's assessment of the relative benefits. The extended runway can also accommodate 740,000 air transport movements, but it was not assessed on this basis."

Air transport movements are any landings or takeoffs that transport passengers. Stanwell added that Heathrow Hub feels its complaints to the DfT and Airports Commission have been ignored and that the expansion decision is not being made using all of the available information.

Heathrow Hub director Jock Lowe added: "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."

MPs are expected to vote on the proposals for the third runway within two weeks. If approved, it will pass to a public consultation and judicial review before the government can give planning permission.

The Labour Party today (20 June) dropped its formal support for the planned third runway, citing its environment impact and fears that its benefit would not be felt across the country.