preservation laboratory, where stem cells were kept for use in transplants in children whose bone marrow has been damaged during chemotherapy.

Concerns were first raised when Sophie Ryan-Palmer, 12, who had acute lymphoblastic leukaemia, failed to make progress after her transplant in June 2013, which involved using a donors stem cells rather than her own.

She had been diagnosed with leukaemia at the age of two and had undergone three previous transplants. She began fundraising for cancer charities when she was six.

By October last year the hospital had identified that a higher than usual proportion of eight children who had undergone stem cell transplantation between March and August had suffered what doctors call delayed engraftment. But by the time it stopped freezing stem cells on site at its base in Bloomsbury, central London, and launched an investigation, three of the eight had died.

Ryan Loughran, aged 13 months, from Bournemouth, died on July 10. Sophie, from Sunbury in Middlesex, followed on July 17. Katie Joyce, from Hertfordshire, died on October 6. A fourth patient, Muhanna al-Hayany, aged five, died in August this year. He had come from Kuwait to have the treatment. Following the deaths it was discovered that the method used to freeze the stem cells had inexplicably stopped working and that, although still alive, the cells were unable to mature properly.

At the inquest, Katie Beattie, the barrister representing Katie Joyces family, questioned whether the girls transplant in August should have been suspended, knowing Sophie and Ryan had died the previous month. Great Ormond Street went ahead even though there was plenty of time to stop it, she said.

Doctors from the hospital told St Pancras coroners court that they regretted not halting transplants sooner and Katies life might have been saved if they had. But they said they believed they were doing the right thing by continuing with the transplants because cancer doesnt wait.

Great Ormond Street said it has since overhauled its procedures to prevent further incidents, but is still investigating why the freezing process stopped working.

A spokesman said: Before giving our patients any frozen cells we carried out tests, which are standard across most laboratories in the UK, to ensure they were alive and viable. All of the samples passed these tests, so there was nothing to suggest there was a problem at this stage.

The coroner, Mary Hassell, is expected to deliver verdicts on all four deaths on Tuesday.

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Stem cell failure 'led to children's deaths' at Great Ormond Street