The backers of a rival plan for Heathrow expansion have begun a formal legal challenge to the government’s proposals for a third runway.

Heathrow Hub has put forward a plan to expand one of the airport’s two existing runways claiming it is more affordable and less damaging to surrounding neighbourhoods.

Sky News reports is has seen a ‘pre-action’ letter sent by law firm DAC Beachcroft acting for Heathrow Hub which it claims paves the way for a judicial review of the approval of the £14 billion third runway.

The legal challenge accuses the Department for Transport (DfT) of not allowing scrutiny into the decision-making process on the third runway.

It challenges the DfT’s National Policy Statement, which ended a political stalemate over the third runway in June, setting out five grounds on which it should be quashed.

Heathrow Hub is funded by a hedge fund manager who‎ financed a search for MH370, the missing Malaysia Airlines aircraft.

Sky News quoted sources that said Heathrow‎ Hub had engaged Martin Kingston QC, a planning expert at No5 Chambers, and Robert O’Donoghue, a competition law expert from Brick Court.

The third runway potentially faces another legal challenge backed by five London councils with the support of London mayor Sadiq Khan and Greenpeace.