The BBC's Peter Holland: "Police will speak to every member of staff at the dentists"

The girl, Karla Rose Selley, from Leyland, Lancashire, collapsed shortly after having a tooth removed under general anaesthetic at Towngate Dental Surgery.

Karla had gone to the surgery with her mother Tracey and her grandmother.

She developed breathing difficulties soon after the tooth was removed.

She was taken to Chorley Hospital, but later died after attempts to revive her failed.

Family devastated

Karla had just returned from a holiday to Cyprus with her mother and Tracey's partner Stephen Armstrong.

BBC North of England Correspondent Rishard Wells: "An investigation is underway"

Mr Armstrong said: "She'd had a tooth out before we'd gone on holiday because she had an abscess. Then when the abscess flared up again the dentist booked her in for an appointment when we got back.

"But when she saw the needle she went hysterical. She was absolutely terrified and the dentist didn't want to put her off because she was so young so it was arranged for an anaesthetist to come from Preston.

"Tracey took her in and she went to sleep and that's all we know - she didn't wake up. It's just devastating. They were such a close family."

Detective Superintendent Mike Arnold, from Lancashire Police, said: "A post-mortem has been carried out and the cause of death is asphyxiation.

"At this stage we have not been able to establish why she died.

"We will be interviewing members of staff at the dental practice as well as members of the family to establish what caused this tragic death."

A receptionist at the surgery has said no-one is prepared to comment on the case.

Strict limits on anaesthesia

The General Dental Council (GDC) introduced strict limits on the use of general anaesthesia last year following several high profile cases in which patients died while unconscious.

General anaesthesia may now only be administered by a consultant anaesthetist working in a specialist centre with well-established procedures for transporting ill patients to intensive care.

However, the technique is still necessary in certain cases, such as children suffering from serious abscesses, or adults who require multiple extraction.

A GDC spokeswoman said it would only conduct an inquiry if there was a conviction or complaint of professional misconduct.

"At this stage our normal procedure is to wait and see what happens," she said.

"We say dentists must think very carefully before giving any sort of pain control to a patient and we would expect dentists to consider all the options available to them."