Six people died when a recovery vehicle carrying a minibus and its passengers was involved in a collision with a lorry on the M25 motorway in Surrey.
It took more than an hour to free the injured man from the wreckage
The victims were travelling in the cab of the recovery vehicle, police said. A man, also in the cab, is in a serious but stable condition in hospital.
The crash happened at about 0215 BST on the anti-clockwise side, between junctions nine and eight, near Reigate.
The motorway was closed for several hours but reopened just after 1100 BST.
The cab of the recovery vehicle was completely destroyed in the accident, which left wreckage strewn across both carriageways.
The lorry driver was unhurt and is being treated as a witness by police.
Officers do not yet know the destination of the minibus or who it was carrying.
The police and ambulance service believe all those involved were adults.
Ch Supt Rob Price said the crash could be the most serious accident the Surrey force has had to deal with on the M25 since it opened in 1986.
"Some of my officers informed me this morning that we haven't had so many people killed for about 20 years," he said.
"When you put that into the context of some of the busiest roads anywhere, then clearly this is a very serious incident indeed."
Mr Price added: "This is a tragic, tragic day.
"Our condolences go out to all those affected.
"The M25 is one of the busiest, if not the busiest, roads. It has a very good safety record.
"It's probably not appropriate to talk in terms of statistics, but this is a very, very severe accident."
A spokeswoman for Surrey Ambulance Service said the injured man was taken to hospital with "multiple and possibly life-threatening" injuries.
She said it had take paramedics over an hour to free the man from the vehicle.
The RAC and Green Flag said the recovery vehicle had not been working on a breakdown call from one of their members.
The recovery vehicle did not have any livery of any of the national breakdown services - but contractors from other garages can be used to reach stranded motorists.