The victims were declared dead at the scene after the 51-seater vehicle veered off the carriageway between the Hindhead Tunnel and the Thursley exit on the A3 road in Surrey at 11.50pm (BST).
The coach hit a tree, but no other vehicles were involved in the accident, reports BBC News.
A number of passengers were injured and have been taken to hospital for treatment, while one serious case saw a persona airlifted to Southampton General Hospital with "life-changing injuries".
"I understand the road was dry. We cannot see any reason that the road conditions would have had any effect on what occurred," investigating officer Inspector Richard Mallett said.
Surrey Police Assistant Chief Constable Jerry Kirkby added: "This has been declared a major incident and we are working with our colleagues from the other emergency services and other partner agencies to provide a co-ordinated response in line with agreed protocols.
"Our thoughts remain with the relatives of the dead and the injured at this time and we are working very hard to notify their next of kin."
The coach was owned by Merseyside Travel, who confirmed that it left Aintree last Wednesday to travel to Bestival on the Isle of Wight.
The three-day music event took place over the weekend and featured headline performances from New Order, Florence + the Machine and Stevie Wonder.
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