The young boy, who has been named locally as Sebastian Goold, remains in a critical condition at Addenbrooke's Hospital, Cambridge.

Sebastian was on his way home from a rugby tournament in Holt with his team mates when the incident happened on Sunday afternoon.

He was on board a 71-seater coach with his father and fellow members of Stamford Rugby Club. His mother was in a car which was travelling behind the coach.

Sebastian, who suffered serious head and leg injuries, was taken to the Queen Elizabeth Hospital, but was later transferred to Addenbrooke's in Cambridge.

Insp Chris Brooks, from Norfolk police, said: 'There is a very difficult time for the family. We've already interviewed some of the passengers on board as well as drivers who were in the area at the time of the incident.

'As a result of initial enquiries we know just prior to the incident that people were queuing for the toilet. Somehow, the door has opened and the boy has fallen from the moving coach.

'The coach was recovered yesterday and is being kept in a secure compound. The vehicle will be examined by police and VOSA investigators and this is the focus of our enquiry st this stage.

'We will also be speaking to other people who were on board at the time of the incident and it would be inappropriate to speculate on possible causes at this stage.'

The coach involved is owned by tour company Hamiltons, which is based in Kettering, Northants. A spokesman for Hamiltons said: 'Our thoughts and prayers are with the family and we are giving the relevant authorities our full co-operation.'

Ross Thain, mini fixtures secretary with Stamford Rugby Club, said: 'He's in Addenbrooke's, he's in intensive care in a critical condition still, I'm afraid. We are all praying that he survives.'

Club president Steve Fowkes said: 'This is obviously a difficult time for everyone concerned, however I will just reiterate mine and the club's support for Seb, family, team mates and friends.'

Sebastian had been taking part in the Kingswood Festival rugby tournament, which was held over the weekend in Holt. More than 700 children took part.

Organiser Sean Chapman, 44, from Briston, said: 'We were in complete shock and were devastated. It had been a great day and everybody had a great time. Our prayers are with Stamford Rugby Club and little boy's family.'

Mr Chapman said about 40 youngsters attended from the Stamford competed in the contest and half travelled in the coach which was involved in the accident. The other half travelled by car.

Messages of support flooded in on social media. West Norfolk Rugby Club tweeted: 'Our thoughts are with the youngster, his family and everyone at Stamford Rugby Club.'

Jon Salter added: 'Thoughts are with Stamford Rugby Club this morning, wishing your young team mate a speedy recovery.'

Former club member and Harlequins player Louis Grimboldby said: 'Thoughts and prayers are with my old club, Stamford Rugby Club and the boy's family. Awful news. Fingers crossed.'

Anyone who witnessed the incident is asked to contact the Serious Collision Investigation Team at Norfolk Police on 101.