One of four teenagers seriously injured last week in a roller coaster crash at one of Britain's biggest theme parks has had a leg amputated, hospital officials said.

Leah Washington, 17, suffered an amputation above the left knee and a fractured left hand, the University Hospitals of North Midlands Trust said in a statement. Joe Pugh, 18, who was with her on the ride, had suffered two broken knees, the hospital added.

Washington was one of four people seriously injured on the "Smiler" ride at Alton Towers in Staffordshire, central England, last Tuesday when two carriages collided.

"Leah has suffered a life-changing injury and now has many months of rehabilitation ahead of her," Washington's father, David, said in a statement.

"We would like to thank all the emergency services at the scene and all the hospital staff who saved Leah's life."

The park, operated by British firm Merlin Entertainments , was re-opened on Monday, though the Smiler ride and similar rides at other Merlin parks remain closed while an investigation into the crash continues.

Merlin said on Friday increased safety measures had been introduced and that it was supporting those injured.

Shares in the firm have fallen 4 percent in a week on reports that it lost 500,000 pounds ($760,000) a day while the park was shut.