A woman has died following a pile-up involving around 20 vehicles on a major road.

The A40 was forced to close in both directions following the multi-vehicle crash, which stretched for more than half a mile on the A40 near Witney in Oxfordshire, South Central Ambulance service said.

The female driver was declared dead at the scene, while two people were taken to the major trauma unit at the nearby John Radcliffe Hospital in Oxford.

One of the patients, a woman, was fighting for her life, South Central Ambulance Service spokesman David Gallagher said.

A further 10 to 15 casualties were treated at the scene following several collisions at around 8.25am. Two people with minor injuries were taken to hospital.

Images showed the mangled wreckage of several of the cars involved, with debris strewn across the road.

Pregnant mother-of-two, Ami Gaston, from Carterton, was in the car with her partner when she saw the first vehicle crash. She said the accident happened about half a mile from the Witney exit, blocking the right lane.

"A man on his own had skidded and crashed into middle barrier. My partner was driving - we had to swerve out of the right lane to avoid hitting him. His door was open and air bags were released," Miss Gaston said.

"I phoned an ambulance but couldn't stop. My partner wouldn't stop because I am 20 weeks pregnant and didn't want to put us in danger."

She added that the heavy fog meant the situation was "really dangerous", as it was difficult to see any other car until you were "on top of it".

Six vehicles were said to be badly damaged, while a further 10 to 15 were also involved, with the most serious injuries happening in the middle of the pile-up.

The incident is likely to cause major disruption for many people travelling across the country following the Christmas break, as the road connects London to Wales.

Mr Gallagher said: "There was a collision towards the front of the queue and the more serious one about half-way down."

He added three ambulances, three rapid response vehicles and two ambulance officers and an air ambulance car had attended the scene.

By mid-afternoon, the westbound carriageway was reopened by police.

A Thames Valley Police spokesman said: "Thames Valley Police officers remain at the scene of multi-vehicle collisions on the A40, Witney.

"Sadly, a woman was pronounced dead at the scene earlier today."

He added: "The eastbound carriageway will remain closed while officers carry out investigations at the site of the collisions.

"All vehicles which were not involved in the collisions have now been released from the site."

Another eyewitness arrived at the scene later and described the "carnage" as multiple cars swerved and crashed on the busy road.

The woman, who asked not to be named, said she "luckily" saw hazard warning lights ahead as she approached on the slip road from Carterton, and moved on to a grass verge before witnessing the pile-up.

"I saw cars going zooming past me and trying to swerve out the way of the stationary cars but crashing into them," she said.

"I saw one car try to swerve in between a Mini and another car, ending up crashing into the Mini and (flying) up in the air.

"I saw two cars swerve as they braked so hard and go flying into the barrier in the middle of the road ... it was carnage," she added.