Sources have said that the Government will give a steer that it is minded to go with Heathrow this week but the statement will fall short of the final green light as promised by the Prime Minister.

David Cameron is expected to say that a final decision will be subject to the conclusions of a further investigation into environmental impact of a third runway at Heathrow including specifically how it will meet air pollution targets.

The move delays the final verdict on runway expansion until after the next mayoral elections for London in May, which will see staunch anti-Heathrow Campaigner Zac Goldsmith standing for election.

Londoners had been expected to hear within the next few days but now the massive piece of UK infrastructure has been pushed back again.

Howard Davies’s Airports Commission recommended building a new runway at Heathrow, rather than Gatwick back in July after a thorough investigation into the merits of several different proposals to expand airport capacity in the south east.

But the report stipulated that Heathrow’s £18bn third runway plan was only favoured if it could meet stringent conditions on noise and air pollution.

These conditions would include a ban on night flights, legally binding caps on noise and air quality – and legislation to rule out ever building a fourth runway.