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Four people have been seriously injured after two carriages on The Smiler rollercoaster at Alton Towers collided.

According to the West Midlands Ambulance Service, there were 16 people riding in one carriage on the attraction when the crash happened just after 2pm.

It is believed the passengers crashed into a stationary carriage which was empty at the time.

Danny Simm, who was at the Staffordshire theme park when the accident happened, tweeted: "People unconscious, knocked out. Blood everywhere. It really was shocking. Air ambulances landing now, people still stuck."

Lloyd Coller, who was visiting Alton Towers for the day, told Sky News: "I didn’t actually see it but I heard it. It sounded exactly like a car crash – just metal on metal."

Another witness, Tom Whelton, said that The Smiler had also broken down three hours earlier following technical difficulties.

An air ambulance has arrived at the scene with a trauma doctor on board, and everyone on board the ride is receiving triage.

Four road ambulances, accompanied by senior paramedic managers, have also arrived on the scene to cope with the potential casualties.

A spokeswoman added: "Ambulance staff and medics are working quickly with resort staff to gain access to the seriously injured."

In a statement, Alton Towers said: "There has been an incident on the Smiler this afternoon involving two carriages coming together on a low section of the track. One of the carriages was empty and the other had 16 guests in.

The resort’s fully qualified first responders were on the scene immediately to assist with the evacuation which is ongoing, and the area has been closed to allow for access to emergency services vehicles.

"To ensure the safe and timely evacuation of guests, a platform is being constructed. This will take a few minutes but will enable us to proceed more quickly.

"There will be a full investigation once we have recovered the guests, who are our priority."

According to the official website of The Smiler, the £18m ride is the world's first 14 looping rollercoaster.

It opened in May 2013 - and the ride was affected by two incidents within the space of just six months.

Two months after it opened, in July, a metal bolt fell off the ride - leaving a gap in the track. Some 48 people had to be rescued.

Then, in November 2013, passengers sat in the front seats of the ride were hit when several of the rollercoaster's wheels fell off.

The resort claims the ride features "a series of twisted psychological effects including optical illusions, blinding lights and near-misses designed to mess with your mind".