Two lanes are closed on a one mile section of the clockwise M25 in Hertfordshire this afternoon.
The closed lanes are between junction 20 (for the A41) and junction 21 (for the M1), and are a safety precaution while essential repair work is carried out. The clockwise entry slip road at junction 20 is also closed.
Engineers are carrying out repairs to the underside of the Gade Valley Viaduct, which carries the M25 over the railway, Station Road, the River Gade and farmland.
Highways England and its contractors are working to reopen the lanes as soon as it is safe to do so. The repairs, which are taking place underneath the bridge itself, are likely to last well into tonight.
Drivers are being advised to plan their journeys and allow additional time.
A clearly signed diversion route is in place for the entry slip road via the anti-clockwise carriageway to junction 18.