The driver of a National Express coach which overturned at a motorway service station was arrested last night on suspicion of drink driving after the crash which left six passengers seriously injured.

The single-decker, with 33 passengers, was travelling from Birmingham to Stansted Airport when it toppled over on the entry slip road to the Newport Pagnell services in Buckinghamshire. The driver was cut free from the wreckage and arrested by Thames Valley Police on suspicion of drink driving and dangerous driving.

It was the second serious crash involving a coach belonging to National Express, which is due to relocate its national headquarters from London to Birmingham next year. In January, three people died when one of its double decker services from Heathrow Airport to Glasgow overturned on a slip road on the junction between the M4 and the M25.

Yesterday's crash happened at 4pm, two hours after the 777 service left Birmingham's Digbeth coach station. It had stopped at Birmingham International at 2.20pm and Coventry at 2.40pm, and was on its way to Luton Airport before a scheduled arrival at Stansted Airport in Essex at 5.55pm.

A spokesman for South Central Ambulance at the scene said 34 people had been taken off the bus. Six were seriously injured, 11 were walking wounded and 17 were discharged at the scene, he added.

One of the passengers said the coach was "going fast round corners" before the crash. Kirsty Plummer, from Luton, who had got on board at Coventry, said: "I knew something was going to happen, the driver was swerving so I put my seat belt on.

"People near me weren't wearing seat belts and fell on top of me, and they were the ones who were seriously hurt. I smashed my head on the back of the seat and twisted my arm. I was lucky I had my seat belt on."

Witness Raj Bagna said the injured were mainly "walking wounded". He said: "There were just a few we had to physically carry out of the coach."

Eddie Gershon, a spokesman for Welcome Break, which operates the service station, said: "The coach went over on its side after hitting a lamppost and a tree, I've been told, and ended up in the car park.

"About a dozen staff from Newport Pagnell came out and gave initial first aid before the emergency services arrived."

Traffic on the motorway was not affected but access to the services was blocked off.

A casualty bureau, for those concerned a relative or friend was on board, was set up by Thames Valley Police on 0800 056 0146. National Express set up an emergency inquiry line for worried friends and relatives of those on board on 0800 731 2734.

* The chief executive of National Express today moved to reassure the public on the company's safety record.

Richard Bowker said the firm was "absolutely obsessed" with safety and stressed there would be full co-operation with the police following the crash.

The driver, who had to be cut free from the wreckage, was breath-tested at the scene and arrested shortly afterwards on suspicion of drink-driving and dangerous driving. Police are continuing to question him today.

Speaking to the BBC Radio 4 Today programme, Mr Bowker said: "This is now a police investigation. We will fully co-operate with that and obviously we need to learn the detail of this ourselves as quickly as possible."

Put to him that it seemed "odd" that the coach overturned following the clipping of the kerb, he said: "I think what we have to do in these situations is get all the detailed analysis done and not speculate.

"The safety record of the coach is absolutely superb and we need to understand why this did happen."

Mr Bowker went on: "It is so incredibly rare for us to have an incident like this... travelling by coach is far, far safer than travelling by car - it was last week, it will be this week and it will be in the future.

"Once you are driving for National Express, then the standards to which you have to perform are higher in terms of drink than the legal requirement, considerably higher, so we are very tough.

"We randomly test and those random tests mean it is extremely likely that you will be caught by a random test at some point.

"We are absolutely obsessed about safety, we take it extremely seriously."