Plans for a third Heathrow runway have been hit by a report which says part of the M4 will have to be put into a tunnel to meet pollution targets.
Pollution concerns could lead to a tunnel being built for the M4
Environmentalists say the Whitehall study signals the end of the controversial plans, announced in a government White Paper last year.
The report said EU pollution targets meant the M4 would have to be put in a tunnel between junctions three and 4A.
But the government said the plan was just one option being considered.
The report entitled Air Quality Assessments added that motorists could face a £20 access charge for driving to the airport.
It is one of 27 documents which follow the White Paper which, when first published in December 2003, envisaged a short third runway at Britain's biggest airport to be built in the 2015-2020 period provided that strict environmental limits could be met.
John Stewart, chairman of Heathrow anti-noise group Hacan ClearSkies, said: "Even on the wildly optimistic assumption that planes will become 40% quieter over the next 20 years or so, pollution levels around the airport will still be illegal.
"Any responsible government would be dealing with existing pollution levels rather than coming up with madcap schemes like putting the M4 in a tunnel.
"In a sane world this report should kill off any idea of a third runway."
A Department for Transport spokesman said: "The M4 tunnel option was just one of a number of options put forward in documents just published that accompany the White Paper.
"We don't have any plans to put the M4 in a tunnel although we acknowledge that there is much work to be done if the new runway is to meet environmental standards."