Alton Towers has been closed while an investigation continues into a "dreadful" rollercoaster accident that left four people with serious injuries.
Two carriages crashed on the Smiler ride at the Staffordshire theme park on Tuesday, leaving some passengers trapped for four-and-a-half hours.
Two men, 27 and 18, a woman, 19, and a girl, 17, have suffered serious leg injuries.
Bosses have announced that the park will also be closed on Thursday.
The BBC's Danny Savage said it was understood some of those on the ride sustained "life-changing injuries".
Senior paramedic, Peter Howell, said: "The women's injuries were worst, both suffered open wounds and damaged legs; the two men had leg and chest injuries but were less seriously hurt."
A fifth person, a man in his 20s with neck and abdominal injuries, was also taken to hospital and another 11 people required medical treatment at the scene.
The 16 were rescued from 25ft (7.6m) up in the air at an angle of about 45 degrees.
Visitors to Alton Towers reported on social media that the ride had broken down earlier in the day. People at the theme park reported hearing "a loud crash" when the collision happened.
Asked whether human error could have been a factor in what happened, Nick Varney, chief executive of the park's owners Merlin Entertainments, said it was too early to tell.
"Our business is about giving people memorable experiences with the emphasis on safety and yesterday something dreadful happened," he said.
"Those two cars should not have been on the same piece of track. Technically that should not have happened.
"There are braking locks that should stop two cars being on the same section of track and somehow that didn't work the way it was meant to."
Mr Varney said he could not say when the park would reopen.
Earlier he said: "I would like to express my sincerest regret and apology to everyone who suffered injury and distress and to their families. The safety of our visitors is our primary concern. "
Infinity Coaster rides
- The Smiler was the first of three Infinity Coasters built by German manufacturer Gerstlauer
- The other two coasters are at PowerPark in Finland and Erlebnispark Tripsdrill in Germany
- The coaster in Finland, called Junker, has only been open to the public since Saturday and is running as normal today
- Mikko Kiviluoma from PowerPark said: "Our rollercoaster Junker is a brand new model with the latest technology, so as we know it is a different model. Junker was built up during this spring 2015 and tested over 2,000 times before opening. Safety is number one thing in PowerPark and we do normal checks every day for all our rollercoasters and other equipment."
Mr Varney said a full investigation was under way and Alton Towers was continuing to work with the emergency services and the Health and Safety Executive (HSE).
The HSE said it would take action to protect the public if it uncovered "evidence that could affect the safety of other rides at the park or elsewhere".
Thorpe Park, Chessington World of Adventures, Legoland Windsor and Drayton Manor Theme Park all said they were carrying out the "rigorous" safety checks they perform every day.
David Bromilow, operations manager at Drayton Manor Theme Park, said: "Every ride undergoes a rigorous daily safety inspection by our experienced maintenance team covering all maintenance and testing before we open the park to our guests.
"As well as the daily inspection and testing, all rides are inspected and verified regularly by independent inspectors in compliance with the HSE guidelines for safe operation."
Alton Towers' £18m Smiler rollercoaster, which boasts a world record 14 loops, has been closed on two occasions because of safety concerns since opening two years ago.
Asked about previous problems with the ride, Mr Varney said he thought there was "an awful lot of misreporting going on about that".
"Guest safety on those sorts of incidents is not really a major issue in the sense that when you are on a rollercoaster car, the car can't come off the track. When you have a glitch and the ride stops, it's not really an issue of safety to the riders," he said.
Customers with tickets for Wednesday can change them to another day or request a refund through the website.
Asked about the closure, Mr Varney said the park would take "a more measured view of what's going on and whether it was specific to just the Smiler and then take a view about opening Alton Towers".
Following their rescue from the ride on Tuesday, the four people who suffered serious injuries were airlifted to major trauma centres.
In total four people were taken to Royal Stoke University Hospital, including one who has since been discharged, and one person - the 27-year-old man with serious leg injuries - was taken to University Hospital Coventry and Warwickshire.
Five men and six women required medical treatment at the scene.
Following the accident, Merlin Entertainments was the biggest faller on the FTSE 100, with its shares down 3%.
The firm, based in Poole, Dorset, has run Alton Towers since buying out previous owner the Tussauds Group in May 2007.
History of problems at Alton Towers
- A group of 16 journalists were left dangling on the Smiler at a steep angle as they tried out the rollercoaster before it opened to the public
- The ride shut for four days in July 2013 after a piece fell off the track, and 48 people had to be rescued
- In November 2013, it closed for five days after wheels fell off and hit four people in the front carriage. The injured people were looked after by park staff and did not need treatment by the ambulance service, a spokeswoman said at the time
- Riders were also left stranded in the air when the ride ground to a halt at the top of a near vertical section 14 months ago