A lorry driver is in a serious condition in hospital after his vehicle collided with another lorry on the M4 in Berkshire.
The accident, on the westbound carriageway between Chieveley and Hungerford, has caused major delays.
The Highways Agency says it is likely to take until this afternoon for traffic to get back to normal.
Thames Valley Police are appealing for witnesses to the collision, which happened at 2.38am.
No arrests have been made at this stage.
Investigating Officer, Sgt Tony Griffiths of the Joint Operations Roads Policing Unit, said: “We are appealing for anyone who was travelling on the M4 in either direction this morning between the times of 2.30am and 3am near to Junction 13 and 14 of the M4 Westbound to come forward as they may have information to help us with our investigation.
Three fire engines attended along with a heavy rescue unit.
The westbound carriageway remains closed, with severe traffic congestion on both the M4 and A34.
Diversions are in place.