The hard shoulder of the M4 will be converted into a fourth lane, despite a warning it could exacerbate "deadly air pollution" levels.
A 32-mile stretch of the motorway from junction three at Hayes to junction 12 at Theale, West Berkshire, will be widened under the controversial scheme.
Variable speed limits will also be introduced along that section, changing subject to traffic flow, making it what has been dubbed a "smart motorway".
Transport secretary Chris Grayling gave the go-ahead on Friday (September 2) for the measure, which is designed to ease congestion.
Work on the project is scheduled to begin in March next year and to last for five years.
Highways England, which is responsible for motorways across the country, estimates it will cost between £586.4m and £862.4m.
'Allowing traffic levels to increase is an outrage'
In making his decision, Mr Grayling ruled that the change would actually improve safety on that stretch of the motorway.
He recognised concerns about the potential impact on air quality, which he said would be monitored once the fourth lane is operating.
However, Jenny Bates, of the environmental pressure group Friends of the Earth, branded the decision "awful news for local people and the environment".
"Widening the M4 will lead to more traffic, more climate changing emissions and increase air pollution levels that already break legal health limits," she added.
"The requirement to at least monitor the toxic gas nitrogen dioxide is welcome, but allowing traffic levels to increase when they need to be cut to help meet EU legal limits for deadly air pollution is an outrage.
"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."
