Eight people have been killed after their minibus was 'squashed flat' in a serious crash with two lorries this morning.
Four others - including a young child - remain in hospital after the collision at 3.15am near junction 15 southbound at Newport Pagnell in Buckinghamshire.
A FedEx motorist and a second lorry driver have been arrested on suspicion of causing death by dangerous driving. One of the men has been charged with being over the legal alcohol limit.
Police say the minibus was believed to have travelled from the Nottingham area.
'The fire and ambulances services did a really good job.'
Photographs this afternoon show the devastating impact of the crash, with the minibus virtually flattened and the front of the cabs smashed up.
The vehicles were travelling in the same direction when the crash happened, police said.
One of the lorries belonged to FedEx while another was from a company called L M Logistics.
An image of the destroyed FedEx cab shows the windscreen smashed and the front of the bonnet crushed. It was seen being lifted off the carriageway this afternoon.
A witness told MailOnline: 'It's horrific. There is what looks like the remains of the back end of the minibus trapped beneath the FedEx trailer, totally wrecked.
'I don't see how anyone could have survived that. There's wreckage along the grass verge and the third lorry is just past the junction.
He said: 'It looks like the trailer has become detached as it is at right angles to the cab. There are two big rescues vehicles at the scene using cranes. The road is closed to the previous junction.'
Ollie Miller, who witnessed the aftermath of the crash, said: 'It looked horrible, the two lorries were badly smashed and I could see a vehicle underneath one of the lorries. It was totally destroyed. The minibus was basically flat.
'I feel sorry for all those affected.'
Gayle Smith, who drove past the accident, commented on social media: 'We have just passed this accident, travelling on the Northbound carriageway. The accident looked awful. Thoughts and prayers going out to all victims. Our emergency services doing an amazing job!'
Valerie Selby also said the crash was the 'worst she had ever seen'.
She said: 'If you're on the M1 southbound Northampton or heading that way - chose an alternative route ASAP & be patient, the accident that has shut the motorway is the most horrific I have ever seen & I imagine road will be shut all day.'
Tarpaulin has been placed over the scene of the tragedy while dozens of emergency service operatives carry out their investigations.
Emergency services are currently still on the scene and the motorway is closed off between Northampton and Milton Keynes. Motorists are advised to avoid the area while the emergency services work at the scene.
The southbound carriageway between J15 and J14 is currently closed and is likely to remain closed for a significant amount of time
Chief Inspector Henry Parsons from the Joint Operations Unit for Roads Policing, said: 'Emergency services are currently on the scene responding to this collision and road closures are in place.
'We would ask all motorists to please avoid the area, use alternative routes and await further updates before attempting to travel on this route. We appreciate motorists' patience while we deal with this serious incident.
'I would appeal to anyone who witnessed the collision and has not yet spoken to officers to please contact us.
'Anyone who has any information which could assist the investigation should call the 24-hour Thames Valley Police enquiry centre on 101 quoting URN 214 of 26/8.'
Congestion has been building from junction 15a at Northampton Services today as thousands were due to descend on the M1 motorway for the weekend.
Around 4.4million motorists are expected to take to Britain's roads today, as families travel up and down the country.
And with unprecedented levels of rail repairs closing Euston, Waterloo, Charring Cross and Liverpool Street, many are left with no choice but to take to the roads.
Northamponshire Police has asked people coming to the British Moto GP at Silvestone today to allow more time for their journey.
The Highways Agency has advised anyone travelling south to exit at junction 15 and take the A508 southbound through Roade and Grafton Regis to the A508/A5/A422 roundabout at Deanshanger.
They can then exit onto the A5 and continue to the A5/A509 Interchange.
A million visitors were due in Cornwall this weekend, with 300,000 visitors expected in Brighton across today and tomorrow, with 200,000 in Blackpool, 150,000 in Bournemouth and 150,000 in Great Yarmouth.
A Brighton spokesperson said: 'It's our busiest weekend of summer.'
Traffic jams were expected on coastal routes including the A23 to Brighton, A31 to Dorset, A30 to Cornwall and M55 to Blackpool.
The warmer weather is boosting 17 million Brits on UK holidays or daytrips this weekend.
According to data from VisitEngland, data showed 7million are staying away overnight, with 10million having a day out.