A lorry driver has been arrested after a horror crash on the M25 that left three road workers injured, one of them critically.
The road was closed in both directions between junctions 23 and 25 this afternoon after the incident.
It has since reopened but long tailbacks built up as diversions were put in place. Delays have continued into the evening rush-hour.
Police said the lorry driver was being held in custody on suspicion of causing injury by dangerous driving.
Four people in total were injured in the crash, three of whom were road workers carrying out repairs on the M25, a Metropolitan Police spokesman said.
Police were called at approximately 9:24am and ambulance and fire fighter staff also attended.
One man was taken to a hospital in Cambridgeshire by Hertfordshire Ambulance Service where he remains in a critical condition.
A second man was taken to a central London hospital by London Air Ambulance where he remains in a serious condition. A third man is being treated at a central London hospital for a leg injury. The fourth man received treatment at the scene for minor injuries.
The incident led to the closure of the motorway between junction 23 at South Mimms in Hertfordshire and junction 25. Long tailbacks built up as diversions were put in place. Later, the section between J23 and J24 was open but delays continued into the evening rush-hour.
Police said the road workers were all working on the M25 which is being widened in that area.
The police went on: "Police have arrested a man on suspicion of causing serious injury by dangerous driving following the lorry overturning between junctions 24 and 25 on the M25. He remains in custody at a north London police station."