Comments at Prime Minister’s Question Time yesterday by David Cameron are being interpreted as paving the way for a U-turn on blocking a third runway at Heathrow.

He refused to rule out a change of heart when replying to a question by environmentalist and Richmond Park Conservative MP Zak Goldsmith.

While restating the coalition’s position, Cameron said: “We must not be blind to two important considerations: how we expand airport capacity overall and how we ensure Heathrow operates better.”

Government insiders confirmed to the Financial Times that both Cameron and chancellor George Osborne had an open mind on expanding Heathrow, although any change of policy would not come before the next general election in 2015.

Cameron left open the prospect that the Conservatives would campaign at the election in favour of expanding Heathrow.

He reportedly believes there is an urgent need to develop London’s air links with emerging markets but has cooled on plans for a new hub in the Thames Estuary promoted by London mayor Boris Johnson due to its high potential cost and length of time to construct.

A third Heathrow runway, which could open in six to 10 years, would cost an estimated £10 billion against £50 billion for the so-called ‘Boris island’ which would not open before 2028.

A Department for Transport consultation document on aviation policy is due this summer but final decisions are expected until another transport department document is published next spring.