A migrant who hid under a lorry travelling from France has been killed trying to get out while the vehicle stopped on the M25.
He was one of two men from Sudan thought to have climbed under the lorry at Calais on Friday, police said.
They decided to leave the vehicle after becoming “extremely cold” after the lorry arrived in England.
But one of the men is believed to have fallen beneath the rear wheels when it started to move off again, police said.
The other man was not injured and was detained by police on suspicion of immigration offences.
Police want to track down the driver, who failed to stop.
The men are believed to have climbed underneath the lorry – a “foreign HGV” – at the French port of Calais on Friday morning.
The vehicle went through the Channel Tunnel to England, then headed towards London and joined the M25. The accident occurred at about 19:00 GMT on Friday between junctions 24 and 25 of the motorway.
A Metropolitan Police spokesman said: “While the HGV was stationary in lane one of the motorway, one of the men worked himself free and clambered down the nearby grass verge.
“The second man attempted to do the same, but the lorry began to move off and he is believed to have fallen beneath the rear wheels of the lorry, suffering fatal injuries.”
A member of the public pulled the man to the edge of the carriageway but then drove off.
A second member of the public then stopped and offered assistance.
Paramedics tried to revive the man but he died at the scene.
Traffic on the M25 and surrounding routes suffered major delays due to a road closure.