A man has been arrested in connection with a fatal crash on the M1 near Watford this morning.

The 25-year-old from East London has been arrested on suspicion of causing death by dangerous driving, Hertfordshire Constabulary confirmed this afternoon.

The high-speed crash, involving two black BMWs, took place just south of Watford at around 2.40am and caused the deaths of the driver and rear seat passenger in one vehicle.

The men, aged 22 and 24 and both from East London, were pronounced dead at the scene.

Two other passengers in the vehicle, both men, suffered serious injuries and have been taken to a hospital in London.

The four men in the second car were also injured and were taken to hospital.

Meanwhile, attending firefighters and a passing motorist have described their horror at what they saw.

Fire crews from Garston, Watford, Borehamwood and Bushey were called to the accident between Junction 5 (Watford) and Junction 4 (Edgware) at about 2.48am.

They were joined by police and paramedics and a rescue support unit from St Albans.

Dave Elliott, Watford Red Watch Commander, said: "It was a very serious incident and particularly harrowing.

"We’ve had a very long morning, trying to assist, from the initial callout at 2.48am until we left at about 6am.

"When we arrived at the scene, we discovered five people trapped in the two cars. We had to use two hydraulic cutters on the BMWs to free them.

"The incident was a multi-agency rescue, and everyone did as much as they could to sort the situation out as quickly as they could. It’s been a very tough morning."

A motorist, on his way to work, described what he saw as he drove along the M1.

He said: "The crash itself was about 200 metres from junction 5. When I drove past at about 8am, there was a large green screen in place and a police officer waving motorists by, to stop them from looking. There must have been about ten police cars there.

"I saw two cars involved in the crash, but they were unrecognisable, they had been so seriously damaged, they looked as though they had been crushed. It must have been a horrific crash."

The northbound carriageway of the motorway was closed until 6.20am.

The southbound carriageway was closed between Junction 5 at Watford and Junction 1 until around midday, causing traffic disruption in both directions.

Disruption was also reported on surrounding roads including the M25, A414 and A41 as a result of the crash.