The driver of a coach which crashed killing two people has been arrested on suspicion of causing death by dangerous driving, police said today.
After the horrific crash on a motorway slip road near Heathrow airport, survivors had limbs amputated at the scene as paramedics fought to save their lives, hospital bosses said.
And one survivor of the National Express crash described how his wife " flew past him" as the coach overturned.
The 40-year-old driver was arrested by officers shortly after 11.30am today, and will be interviewed at a Berkshire police station, Thames Valley Police said.
The coach has been taken to a garage in Oxfordshire where it will be forensically examined as part of the investigation.
According to police inquiries, 69 people, including the driver and his co-driver were on the coach.
Thames Valley Police said one of the fatalities was a woman in her 60s from Dundee.
Gordon Welsh, 73, was travelling home to the Isle of Skye with his wife Audrey, 70, after celebrating their 50th wedding anniversary with family when they were caught up in the incident.
Mr Welsh, who fractured his shoulder and hurt his lower back, told relatives he saw his wife "fly past him". After the crash he could only reach out and touch her hair to let her know he was there.
He was able to walk away from the wreckage, but Mrs Welsh, who suffered a dislocated hip and fractured ribs, was trapped and had to be taken out by ambulance crews.
Both were taken to St Peter's Hospital, Chertsey.
Family members described how the couple remembered the coach clipping the kerb before swerving to the left and toppling on its side. They were sitting upstairs, on the left-hand side and did not take the full impact.
Daughter Jackie McDonagh said her father is likely to go home today, but doctors found fluid between her mother's lungs and chest wall and said she will be in hospital for around four days.
David McVittie, chief executive of Hillingdon Hospital, which took the majority of the casualties, described this morning how passengers underwent amputations at the scene.
He said: "Staff have treated serious head injuries, serious fractures, internal bleeding and things like that - pretty horrific.
"People were having to have amputations on the spot to be freed from the wreckage."
Mr McVittie said one of the seriously injured patients was now in the hospital's intensive care unit after being operated on and another who was also seriously injured was going into theatre later today.
A man and a woman were confirmed dead by Thames Valley Police early this morning after suffering serious injuries.
The coach was travelling from London to Aberdeen when it overturned on the slip road leading from junction 4B of the M4 eastbound to the M25 junction 15 at 11.45pm.
Firefighters used hydraulic cutting gear, airbags and thermal imaging cameras to help rescue seven people.
The injured were take to six hospitals in west and central London.
The majority were taken to Hillingdon Hospital.
A spokesman said 37 casualties were taken to its A&E department. Four were seriously injured and two of those subsequently died.
He said: "Of the two remaining seriously injured, both are in a critical condition.
"Nineteen people were treated and released from the hospital in the early hours of this morning and the majority have continued their onward journey.
"The remaining casualties remain at the hospital with mainly trauma injuries, broken and dislocated limbs, back injuries and cuts and bruises."
One of the injured was a 14-year-old boy and one patient has undergone an arm amputation.
A further 16 casualties, 10 men and six women, were transferred to Charing Cross Hospital in Fulham, west London.
One of those was later transferred to another hospital, and three are "giving doctors cause for concern", a spokesman said.
The others were expected to be discharged this morning.
Seven were taken to West Middlesex Hospital in Isleworth, west London. Among them were a one-year-old and a two-year-old child.
A spokeswoman for the hospital said the casualties started arriving at 3am. All arrived by ambulance and five have since been discharged. One of the women still being treated is in a critical condition while the other is stable.
Four of the casualties were being treated at St Peter's Hospital and the condition of all four was described as "serious".
Three of the four passengers are from Scotland, a hospital spokesman said.
Three were described as serious but stable: a 70-year-old female, a 73-year-old male and a 40-year-old male from Poland. A woman in her 80s was in a stable condition.
Three adults and a child were taken to Wexham Park Hospital, Slough, by ambulance last night.
The child has now been transferred to St Mary's Hospital.
Of the three remaining, there were two men - one stable and one in a critical condition - and one woman, who was stable.
Two children, a boy and a girl, were being treated in the paediatric intensive care unit at St Mary's Hospital, a spokesman said.
National Express said it was providing hotel rooms, taxis and assistance to passengers and relatives. Members of its senior management team were at hospitals across London today.
Aerial shots of the crash scene this morning showed the coach with a black tarpaulin over its damaged roof, shielding the scene inside from public view.
At 9am it was hooked up to the back of a lorry and removed from the closed motorway.
Police collision investigators and emergency services were continuing their work at the scene to establish the cause of the incident and to ensure the road was cleared as quickly as possible.
Thames Valley Police issued an emergency telephone number - 0845 8505505 - which is also the number for witnesses who have not already spoken to police.
National Express customers can contact a dedicated 24-hour emergency number on 0121 625 1278.