Four people have been taken to hospital after a car crash on the M1 this morning.

Hertfordshire Constabulary said a Renault Clio and a Vauxhall Astra collided before the exit slip road for the M25 on the south-bound M1 at around 9.40am this morning.

One patient has been airlifted to St George's Hospital while three people have been taken to Watford General Hospitial.

The motorway will be closed between junction seven and six while police carry out investigations into the crash.

Trapped traffic is being reversed back to J8. There are solid queues of several miles approaching the closure with delays of well over an hour.

The diversion is the eastbound A414 to the Park Street Roundabout at St Albans, then the southbound A405. There are long delays on this route.

Sergeant Colin Jenkinson, from the Bedfordshire, Cambridgeshire and Hertfordshire Roads Policing Unit, said: “This is an extremely serious collision which has caused serious injuries to those involved. I would urge anyone who may have seen this incident or either car prior to the collision taking place to come forward.

“The M1 will remain closed between Junction Seven and Junction Six southbound whilst we conduct our investigation. I would ask motorists to avoid the M1 in this area and to make alternative travel plans or find an alternative route as the road is expected to be closed for some time.

The East of England Ambulance Service sent three ambulances, an ambulance officer, a rapid response vehicle and the air ambulance also attended.

A hazardous area response team was also sent by the ambulance service to the scene. This team is a described on the ambulance website as a highly skilled team who can work in difficult environments and allows ambulance crews to "provide emergency patient care faster at the scene of a major incident".

Anyone with information is asked to contact Hertfordshire Constabulary on 101 quoting ISR 175 of February 22.