The M25 anti-clockwise is closed near Bricket Wood, between junction 21a (for the A405) and junction 20 (for the A41), near Kings Langley, for urgent bridge repairs.

As a result, there are severe delays on the M25 anti-clockwise and long delays on M1 southbound approaching this closure.

Motorists are advised to allow plenty of time for their journey and may wish to consider using an alternative route if possible.

Traffic from the southbound M1 onto the anti-clockwise M25 will be restricted to one lane only and diverted off at the M25 junction 20.

A routine inspection yesterday afternoon (Tuesday) on the Gade Valley Viaduct found an isolated crack on a small section of the structure underneath the anti-clockwise carriageway.

The viaduct carries the M25 over the railway, Station Road, the River Gade and farmland.

Since the crack was discovered, engineers have been working around the clock to prepare the repairs, which are now taking place.

The repairs, which are taking place underneath the M25, are likely to last into early this afternoon (Wednesday).

A Highways England spokesperson said: “Safety is Highways England’s top priority. During a routine inspection yesterday a minor crack was discovered on the Gade Valley Viaduct near to the M25 junction 20.

“The closures are in place to allow repairs to be carried out as quickly as possible to stop the crack from growing and maintain safe journeys.

“We would like to thank drivers in advance for their patience and reassure them that we will be working as hard as we can to reopen the M25 as soon as safely possible.”

The anti-clockwise M25 is closed between junctions 21a and 20 with a signed diversion following the square symbol via the A405 and A41.

Drivers wishing to travel onto the northbound M1 will be diverted via the A405 and A414.

Traffic from the southbound M1 onto the anti-clockwise M25 will be filtered down into one lane and diverted off at the M25 junction 20 to use the roundabout and re-join the M25.

Drivers are advised to plan their journeys and consider avoiding the area if possible.

Highways England provides live traffic information via its website (www.highways.gov.uk/traffic-information), local and national radio travel bulletins, electronic road signs and mobile platforms, such as Android and iPhone apps.