Major delays have been reported on the M25 clockwise in Hertfordshire.
It is believed the problems have been caused by cracks being discovered in a bridge between Junctions 21A in St Albans and 20 near Kings Langley..
Engineers are currently working on the structure
Repair work is expected to last into early afternoon.
A Highways England spokesman 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.”
Drivers are being advised to plan their journeys and consider avoiding the area if possible.