A British woman has been found dead in the stairwell of a US hospital where she went missing as a patient more than two weeks ago.

Lynne Spalding, 57, was being treated for an infection at San Francisco General Hospital after she arrived in a thin and frail condition on 19 September.

Her body was only found 17 days later, after a member of the hospital’s engineering staff ran a routine check on a rarely-used external stairwell.

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Officials say they are now investigating how Ms Spalding got out to the stairs unnoticed, with the door she used normally alarmed and locked from the outside.

Speaking on behalf of Ms Spalding’s family, her friend David Perry said her disappearance had been a “nightmare” and demanded an explanation.

Mr Perry said: “Lynne Spalding died alone, in the stairwell, at one of the finest medical institutions in this country.

“I hear that the San Francisco Sheriff's Department initiated a search. Well, evidently it wasn't a very good one. I think there are a lot of questions to be asked.”

Ms Spalding’s friends and relatives spent days searching the streets of San Francisco and putting up flyers, because they were “under the assumption that San Francisco General had been searched and Lynne was not here”.

Todd May, the San Francisco General Hospital chief medical officer, said: “What happened at our hospital is horrible.

“We are here to take care of patients, to heal them, to keep them safe. This has shaken us to our core. Our staff is devastated.”

Ms Spalding was first reported missing from her room two days after she arrived for treatment. Just before she disappeared she was described as being in a fair and improving condition, although appearing disorientated, the BBC reported at the time.

Assistant sheriff Paul Miyamoto said: “All of us are committed to learning what happened and ensuring it never happens again.”

The Foreign Office has said it is aware of the death of a British national and that it is providing consular assistance to the family.