Plans to convert the M4 hard shoulder into a fourth lane of traffic have been given the go-ahead by the Government.
The proposals would see a 32-mile stretch of the highway widened from three to four lanes from Hayes, west London, to Theale, Berkshire.
This stretch of road, from junction three to 12, would also be subject to variable speed limits under the scheme.
Transport Secretary Chris Grayling, who granted planning permission for the scheme, said there is a "critical need to improve the existing national road network".
In his letter of approval he said the plans will "increase capacity, improve traffic flow and reduce journey times, thereby supporting economic development".
Environmental and transport groups are outraged at the decision and claim having no hard shoulder will be a hazard for motorists.
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.
"This is just expanding motorways on the cheap."
She added: "We need investment in alternative options, including better rail, to give people choice in their journeys."
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.
"Motorway widening is not the solution to our congested roads as more traffic just makes it worse, it's time to send UK transport in a new direction to protect our planet and our health."