A passenger who survived the National Express crash has described how she was flung across the coach as it slid on its side.
Fire crews had to cut passengers from the wreckage
Jean Cornwall said she escaped with only whiplash injuries after landing on her back on the double decker's window.
She said she was saved because the safety glass remained intact.
Two people died and 23 are seriously ill after the crash on an M25 slip road near Heathrow. The coach driver was arrested and later released on bail.
Ms Cornwall, from Australia, said as the Neoplan Skyliner vehicle came to rest she was left bearing the weight of a man who was bleeding from a large head wound.
Ms Cornwall said: "The clothes that I had on were really quite hideous - they'll have to be thrown away - they were really quite hideously stained with his blood."
She was travelling to Kintyre in Scotland with her 19-year-old daughter Kathy, who lives in London.
The pair were on the upper deck of the bus along with 65 other passengers as it set off from London Victoria on Wednesday night.
However, less than two hours later, the coach went out of control on the slip road.
She said: "I didn't have a seatbelt on and somehow or other I landed on my back on the window on the right hand side of the bus.
"I must have hit the window at the same time that the bus hit the pavement because I landed on my back on that window but I didn't go through and it was actually the safety glass which cushioned me against the sliding.
'Could not see'
"The bus then proceeded to slide along the road before it came to a halt. It was actually that window that saved me from getting well shredded.
"I could feel scraping along the road. I was aware from before the bus came to a halt there was shouting and panicking ... it seemed quite dark.
"I actually had some baggage over my face that I wasn't aware that it was baggage... I was just aware that I couldn't see anything.
Chris Toner, who died in the accident
"I couldn't move I was pinned down. I had a man on top of me - he was resting his head in my stomach area, and his head was bleeding quite copiously."
"The clothes that I had on were really quite hideous - they'll have to be thrown away - they were really quite hideously stained with his blood."
As well as the 67 passengers, there were two drivers on board the service.
Chris Toner, 76, from Monifieth, near Dundee, was one of those killed in the crash. A three-year-old boy and a baby girl are being treated at St Mary's Hospital in London after losing limbs.
National Express, which has withdrawn its 12 double-decker coaches from service, said there was no suggestion of any stability problem with the coaches at this stage.
Stagecoach said it was carrying out "precautionary checks" on all 50 Neoplan Skyliner vehicles in its fleet following the crash.
It runs a fleet of 25 of the vehicles on its Oxford Tube service to London and on the company's inter-city coach service megabus.com.
Paul Bunting, National Express chief executive, said: "As a precautionary measure, we felt it necessary to temporarily withdraw the fleet as soon as possible.
"We will ensure this process is thorough and comprehensive as part of our commitment to passenger safety."
The Neoplan Skyliner has, by law, been fitted with a speed limiter restricting it to 62mph and all the ones owned by National Express have seatbelts fitted.
All double-decker vehicles undergo Department for Transport safety tests to show they can tilt 28 degrees with the top deck full and the bottom deck empty, without tipping over.
In 2003, 28 people died when a similar double-decker coach overturned in France.
The coach involved in the M4 incident has been moved to a garage in Oxfordshire where it will be examined by accident investigators.
Police have confirmed no other vehicle was involved.