Mr Johnson said he feared there was a "stealthy U-turn" under way and vowed to fight the party leadership if it backed a third runway.
He told BBC Radio 4's World at One:
"What I worry about is that we are now seeing a stealthy U-turn being carried out which I don't think is in the interests of London or indeed of the country as a whole because in the end you can expand Heathrow and you can put in a runway...
"Actually it will be a short runway but you have to come back in 10 years time and do another."
Asked if he would campaign against Heathrow expansion he said: "You bet I will, yes."
He said the possibility of a third runway was "being left open" and claimed the commission would not resolve the issue.
More top news
Scotland Yard confirm that two people arrested in connection with the attack on Westminster have been released with no further action
An appeal to support the family of murdered policeman Keith Palmer has reached more than £700,000 in two days.
PC Keith Palmer will be remembered permanently at the National Memorial Arboretum, Home Secretary Amber Rudd has said.