Hayes to Theale M4 hard shoulder will be widened to become fourth lane
Plans to open a 32-mile stretch of the hard shoulder on the M4 motorway to traffic have been approved.
It will see the M4 between Hayes in west London and Theale in Berkshire widened to become a fourth lane with a variable speed limit.
Transport Secretary Chris Grayling said it was a "critical need" and would "improve traffic flow".
Environmental and transport groups said the loss of a hard shoulder would be hazardous and "bad news for motorists".
Giving the green light for the proposal to transform the motorway between junction three to 12, Mr Grayling said in a letter there was a "critical need to improve the existing national road network".
The move will "increase capacity, improve traffic flow and reduce journey times, thereby supporting economic development", he added.
Bridget Fox, sustainable transport campaigner, said: "This is bad news for both motorists and local residents.
"Motorists and breakdown operatives will be exposed to greater risk with the loss of the hard shoulder."
'Already in operation'
Dubbing the move as "expanding motorways on the cheap", she called for investment in the railway and other alternatives.
Jenny Bates, of Friends of the Earth, said: "Widening the M4 will lead to more traffic, more climate changing emissions and increase air pollution levels that already break legal health limits."
In June, a report published by the Transport Select Committee warned ministers against plans to convert hard shoulders into permanent driving lanes citing serious concerns over "all-lane running" schemes.
The scheme is already in operation in sections of the M25, M1 and M6 and plans for the scheme to be used in parts of other motorways - including the M3 and M23 - are in place.