A section of motorway skirting Heathrow Airport is being widened to cope with perennial traffic jams.
Nearly 250,000 vehicles a day use the section – twice the capacity it was designed for 20 years ago, making it one of Europe's busiest roads.
Transport Secretary Alistair Darling gave the green light last year for the M25 to be widened to 10 lanes between the Thorpe interchange, where it meets the M3, and the Heathrow turn-off at junction 14.
From junction 14 to the Thorney interchange with the M4, the road will be widened even further, to six lanes each way.
For the duration of the work, traffic will be limited to 40mph. Balfour Beatty, which is carrying out the £147 million project, has said all existing lanes will remain open at peak times, although their width will be reduced from 3.25 metres to 2.75 metres.