A serious collision involving two cars on the M1 has closed five junctions of the motorway in both directions, police have said.
The accident happened shortly before 2.40am on the southbound carriage between junctions five and four, near Watford in Hertfordshire.
The carriageway is closed in both directions between junctions one and five, from London to Watford.
The southbound carriageway is expected to remain closed until at least mid-morning but the northbound carriageway is expected to reopen by 7am, a Hertfordshire Police spokeswoman said.
The link road from the A41 westbound to the M1 northbound is also closed, Hertfordshire Highways said.
Drivers are advised to use the A1, A40 or M40.
The northbound carriageway between junctions one and five, from London to Watford, has now reopened, Hertfordshire Police said.
The southbound carriageway is still closed between junctions four and five, between Edgware and Watford.
It is expected to be closed until at least mid-morning, a police spokeswoman added.