Rebel Labour MPs - including Hounslow's Alan Keen - have demanded a vote on Heathrow expansion by adding their names to an Early Day Motion.
Around 40, including west London MPs Alan Keen and Andy Slaughter, have rebelled against the party leadership by signing the motion in the same week Gordon Brown hinted his cabinet will approve the third runway in January.
The motion has 106 signatures altogether.
John Stewart, of anti-expansion campaign group HACAN, said: "Although they cannot force a vote a sizable number of Labour MPs, around 40, are now coming out strongly in opposition to expansion and this is certainly an embarrassment for the Government."
The cabinet alone have the power to approve or limit the airport's expansion but the strength of support for the latest motion suggests a vote in the commons would deny BAA their much-longed for third runway.
Both the Liberal Democrats and the Conservative party have already spoken out against further expansion.