Labour accuses David Cameron of 'being bullied by Boris Johnson' over Heathrow expansion as it makes surprise decision to back third runway

Interim leader Harriet Harman calls for a 'swift' decision to be made on where to expand the UK's aviation capacity

David Cameron and Boris Johnson, leaving the Advantage children's daycare nursery in Surbiton (Getty Images)

Labour has accused David Cameron of "being bullied" by Boris Johnson over his refusal to say whether the government will back the recommendation by the Airports Commission to build a third runway at Heathrow.

The Prime Minister said he would announce a decision by the end of the year but Harriet Harman, Labour's interim leader, suggested the Mayor of London's staunch opposition to Heathrow expansion was the reason behind the government's slow response to Sir Howard Davies' three-year report.

Labour surprised the government by calling for a "swift" decision to be made on airport expansion, claiming it was in a "holding pattern above Heathrow and Boris won't let you land".

Although Sir Howard's report chose Heathrow as its preferred location, it left the door open for a second runway at Gatwick, which could prove to be much easier politically for the Prime Minister in the face of fierce opposition to Heathrow expansion from within his own government.

"It looks like the Prime Minister has been overruled by the member for Uxbridge [Boris Johnson]," Ms Harman said at Prime Minister's Questions. "You should tell him he's not the leader of the Tory party yet. You should tell him that," she joked. "Will you stand up for Britain's interests or will you just be bullied by Boris?"

David Cameron branded Isis a 'poisonous death cult' that was 'seducing' young Britons to go and fight for the terrorist organisation in Syria and Iraq. (AFP/Getty)

Mr Cameron laughed off the attack and said it was important not to prejudice any future decision by commenting on the report before he had studied the findings thoroughly.

The eventual decision could be subject to legal challenges in the form of judicial reviews if the decision appears to have been prejudiced by ministers.

He told MPs: "It is important now there is a very detailed report that we study it and I am very clear about the legal position, that if we say anything now before studying the report, actually you can endanger whatever decision is made.

"The guarantee I can give you is a decision will be made by the end of the year."

Sir Howard Davies and the four other members of Airports Commission unanimously concluded that Heathrow’s proposal for an additional runway north-west of the existing pair is the best way to tackle the crunch in capacity for the world’s leading aviation city. It would cost £17.6bn and involve the demolition of 800 homes.

The Prime Minister himself has made a "no ifs, no buts" pledge against a third runway at Heathrow. In 2009 he said: “The third runway at Heathrow is not going ahead, no ifs, no buts.”

The proposals in Sir Howard's report may have changed significantly from those set out at the time of Mr Cameron's pledge, but having witnessed the way voters treated Nick Clegg's broken promise on tuition fees, the Prime Minister will be aware of the damage that can be caused not only to his own reputation but to that of his party if he U-turns on Heathrow.