The accident happened on Friday night on the M25
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.
The driver, who did not initially stop at the scene, has since been tracked down. Police said they believe he was unaware of the incident, and as a result did not pull over at the time. He has not been arrested.
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."Major delays
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.
The number of migrants trying to reach the UK through Calais has increased as people flee humanitarian crises
Nick Harvey, from the charity Doctors of the World, which provides medical care to migrants, said this type of death was all too common.
He said: "It's not just in the UK where migrants are dying or being injured. This man came from Sudan and every part of his journey would have been perilous, arduous. People are exhausted, mentally tired and every stage of the journey people are dying, people are getting injured."
Increasing numbers of migrants have been trying to enter the UK through Calais in recent months.
In November, local officials said there were around 2,500 illegal immigrants at the port, with the majority from Sudan, Eritrea, Ethiopia and Syria.
Numbers are said to have increased by 50% in the past year as people flee from humanitarian crises in northern and eastern Africa and the Middle East.
Earlier this year, the http://www.bbc.co.uk/news/uk-england-26682326BBC recorded footage of migrants sitting on the axles of lorries in an attempt to enter the UK from Calais.
Cameras showed men running towards UK-bound trucks and climbing beneath the vehicles, with drivers oblivious to their actions.
One migrant said at the time that he did not care about the risks involved.
"This country is not good to live in, we have to go to England," he added.