Under the expansion plan the third runway will be built in 2015
There are "clear inadequacies" in pollution safeguards imposed on the planned expansion of Heathrow Airport, London Assembly members have said.
Measures against the impact of a third runway were not "fit for purpose", its environment committee added.
It was also concerned that no single authority would ensure owners BAA and airlines complied with pollution rules.
A Department for Transport spokesman said its environmental criteria for the airport were "robust and relevant".
A new runway would increase the number of passengers from 67 million to 82 million per year, and potentially up to 135 million.
The government gave the plan the go-ahead in January last year on condition that measures were put in place to limit aircraft noise, air pollution and greenhouse gas emissions.
But London Assembly committee chairman Murad Qureshi said: "Our investigation has raised grave concerns about some of these safeguards, including clear inadequacies in approaches to tackling air pollution levels around Heathrow.
"We would also question whether the suggested noise benchmark is fit for purpose and if the aviation emissions targets are achievable."
It said BAA's range of measures, which include levies on more polluting aircraft and efforts to reduce congestion on the airfield, were insufficient.
The committee also said more action was needed to reduce road traffic, which contributes to the airport's pollution.
It called for a joint government and BAA plan for improving air quality and said a failure to meet EU pollution targets could result in fines totalling hundreds of millions of pounds.
The Department for Transport said a new Heathrow runway would secure jobs and ensure Britain remained a place where the world could come to do business.
Its spokesman said: "However, in giving support for a third runway at Heathrow, the government was absolutely clear that new capacity will only be released once strict air quality and noise conditions are shown to be met and on the basis of independent assessment.
"An enforcement framework for this is being developed and we will consult on our proposals later this year."
In December the government's official climate watchdog Committee on Climate Change said Heathrow expansion was "entirely consistent" with its target to reduce aviation emissions to 2005 levels by 2050.
The Department for Transport spokesman said: "This target helps give the UK one of the toughest emissions regimes for aviation of any country in the world."
The runway is due to be built in 2015 and Conservatives and Lib Dems are among those fighting the £7bn plans.
Anna Jones from environmental charity Greenpeace said: "It's simply impossible to bolt a new airport the size of Gatwick on to the present airport at Heathrow and not breach noise and air pollution regulations.
"It's time for BAA to scrap Heathrow expansion. A new runway would be too noisy, too dirty and pose too much of a threat to the climate."