Strict new targets, agreed as part of the package of environmental safeguards for the third runway, limit emissions from aviation to their 2005 levels by 2050.
But the latest CO2 forecasts from the Department for Transport, released this month, show that the only way to reduce emissions would be to stop expansion at all other airports – including the planned second runway at Stansted.
Stephen Joseph, executive director of Campaign for Better Transport, said: "Just two weeks after transport secretary Geoff Hoon’s announcement of these safeguards and already they are starting to fall apart.
"At the same time as he gave a confident assurance to Parliament that the industry would clean up its emissions, his own department was issuing a dire forecast of ever-rising CO2 emissions. Either the minister has not done his sums properly or he is so determined to expand Heathrow that he is willing to cap capacity at every other airport to do it."
The Department for Transport rejected CBT's claims.
"The aircraft industry, through its Sustainable Aviation initiative, has already produced a roadmap setting out how this target could be met. The Government has asked the Climate Change Committee to advise on the best basis to take this roadmap forward," said a spokesman.
"This roadmap sets out how it is possible to bring aviation CO2 emissions below 2005 levels by 2050 while still providing for the growth forecast by the Government."
The lobby group has timed the release of its findings to influence today's Conservative motion condemning Heathrow. More than 20 Labour MPs are expected to rebel against the Government today.
The Campaign for Better Transport has contrasted Hoon's speech to parliament with documents released by his own department.
Announcing the government’s decision to support a third runway at Heathrow, Hoon said: "I can announce that we will establish a new target to get aviation emissions in 2050 below 2005 levels and I have asked the Committee on Climate Change to advise on the best basis for this development...
"Any future capacity increases at Heathrow, beyond the decision I have announced today, will only be approved by the Government after a review by the Committee on Climate Change in 2020 of whether we are on track to achieve the 2050 target that I have announced."
This target was not included in the report which accompanied this announcement.
The government's UK Air Passenger Demand and CO2 forecasts, published on 15 January, showed that emissions in 2005 were 37.7 million tonnes of CO2 and are expected to grow to 53-65 million tonnes by 2050.
By 2030, Heathrow alone is expected to emit 23.6 million tonnes of CO2. Total emissions from the five London airports, assuming expansion at Stansted and City but not at Luton or Gatwick, are 31.6 million tonnes.
Allowing expansion at Heathrow but not allowing expansion at Stansted brings the total emissions for London's airports to 30.3 million tonnes. That leaves just 7.2 million tonnes of CO2 for all the other airports: below the 2005 figure of 7.8 million tonnes.