Alton Towers amusement park will remain closed tomorrow while an investigation into a "dreadful" accident on a rollercoaster which left four people with serious injuries continues.

The accident happened at around 2pm on the Staffordshire resort's 50mph Smiler ride when two carriages crashed on a low section of the track.

Two males aged 27 and 18 and two females aged 19 and 17 suffered serious leg injuries in the crash, while the other 12 occupants - six men and six women - required triage.

Nick Varney, chief executive of Merlin Entertainments, which runs the resort, said: "This has been a terrible incident and a devastating day for everyone here.

"I would like to express my sincerest regret and apology to everyone who suffered injury and distress today and to their families.

"The safety of our visitors is our primary concern. The park will remain closed until we understand better the cause of this dreadful incident.

"I would like to thank the emergency services for their swift and effective response today - they were outstanding.

"A full investigation is now under way and we will continue to work closely with the emergency services and the Health and Safety Executive."

An Alton Towers spokeswoman said that the park hopes to open again on Thursday and customers with tickets for tomorrow could seek a revalidation or a refund through the park's website.

Those who suffered critical injuries were airlifted to major trauma centres after the 16 occupants were rescued 25 feet up in the air at an angle of about 45 degrees.

A West Midlands Ambulance Service spokeswoman said: "All four were given advanced trauma care, pain relief and immobilisation and were each carefully extricated from the ride and onto the platform before being lowered to the ground.

"The 27-year-old male was then airlifted to University Hospital Coventry and Warwickshire whilst the others were flown to Royal Stoke University Hospital for further emergency treatment.

"The remaining 12 occupants, six women and six men, suffered less serious injuries.

"They were released one at a time over a time period of four hours and lowered to the ground in order for a further assessment of their condition.

"One of the 12, a male in his 20s was treated for neck and abdominal injury and was taken to the Royal Stoke University Hospital by land ambulance for further assessment and treatment."

The 16 occupants' ordeal lasted over four hours, with their evacuation taking until until 6.35pm.

A full investigation with the involvement of the Health and Safety Executive is under way at the park.

The £18 million rollercoaster, which boasts a world record-breaking 14 loops, has been closed on two occasions because of safety concerns since opening two years ago.

In July 2013 it was closed after reports that a bolt was seen to have fallen from the ride and in November that year the rollercoaster was closed after plastic guard wheels came loose and hit front-row riders.

During a previous scare, 16 journalists were left stranded on the Smiler for around 30 minutes during a preview ride before it opened to the public in May 2013.

Following today's accident, Merlin Entertainments was the biggest faller on the FTSE 100, with its shares down 3%.

The firm, which is based in Poole, Dorset, has run Alton Towers since buying out previous owner the Tussauds Group in May 2007.

It is the world's second-largest visitor attraction operator behind Disney and runs 105 attractions, 11 hotels and three holiday villages in 23 countries.

Alton Towers divisional director Ian Crabbe admitted to reporters that he had not been alerted to issues that the Smiler reportedly suffered earlier in the day.

He told a press briefing at the resort: "This ride has had its issues to start with, which is fairly normal for a big rollercoaster.

"I think it would be unfair to reflect on those issues, which were originally part of the ride when we first opened it, and say they are connected to this incident, which I don't believe they are."

Four air ambulances were involved in the rescue effort, together with four ambulances and several senior paramedic managers.

Staffordshire Fire and Rescue said it had sent four pumps, a rescue tender, rope rescue and an aerial ladder to the incident.

A ramp was built to help emergency services reach the occupants of the ride, who were approximately 25 feet up in the air at an angle of around 45 degrees.

Danny Simm, a musician and songwriter who is also a radio presenter on 96.5 Bolton FM and who witnessed the aftermath of the accident, tweeted: "People unconscious, knocked out. Blood everywhere. It really was shocking."

Lucy Farrugia said: "Smiler broke down when I was on it this morning and now it's crashed. Hope everyone on it is OK, saw the air ambulance arrive."

Sophie Underwood, who was waiting to board the ride, told the BBC: "They had made quite a few announcements to say there were technical difficulties. They were sending coaches around with nobody on them.

"And then they said they had sorted it out so they decided to put people on the coach."