Katie Joyce, left, aged four, and Sophie Ryan Palmer, aged 12, were among the four children who died as a result of complications with transplants. Photograph: Steve Parsons/PA

Four children have died after failings in how stem cells used in life-saving operations were frozen at Great Ormond Street hospital, it emerged this week.

The four, who were between one and 12 years old, were among eight children with cancer whose bone marrow transplants did not work as a result of problems with the freezing process.

Britains best-known childrens hospital has admitted that one of them, four-year-old Katie Joyce, might have survived if it had acted more quickly when problems arose.

An inquest into the deaths this week heard that doctors were initially baffled as to why a decade of success using the procedures suddenly came to a halt in summer 2013. Despite extensive investigations, the hospital failed to pinpoint the source of the setbacks in its cryopreservation laboratory, used for freezing stem cells which were kept there for using in bone marrow transplants in children.

The transplanted stem cells were intended to help the childs bone marrow, damaged during chemotherapy, grow again to maximise the chance of recovery.

At the inquest, lawyers for two of the families whose children died accused Great Ormond Street of taking too long to halt the transplants once staff began having concerns.

The hospital has since overhauled its procedures to prevent further incidents and there are calls for the deaths to lead to tighter procedures around how stem cells are stored at hospitals and research centres across the UK.

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

By October 2013 the hospital had identified that a higher than usual proportion of eight patients who had undergone stem cell transplantation between March and August had suffered setbacks after encountering what doctors call delayed engraftment. It immediately stopped freezing stem cells on site at its base in Bloomsbury, central London, and launched an investigation.

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Great Ormond Street deaths caused by stem cell lab failures, inquest told