Mystery continues to surround the death of a vulnerable, elderly woman who accidentally drank Flash cleaning fluid while in hospital, an inquest heard.

Joan Blaber died on September 23 last year while being treated at the Royal Sussex County Hospital in Brighton.

The city’s senior coroner Veronica Hamilton-Deeley said it was probably “no exaggeration” to say “no-one can actually say what happened,” as she introduced evidence to a jury on Monday.

At the hearing at the Jurys Inn hotel near Brighton Station, she said: “This is incredibly worrying. It could happen again.

“We don’t know how it happened in the first place.”

Mrs Hamilton-Deeley described the inquest as “hugely complex” and “difficult”.

She said: “Even the most rigorous police investigation has not completely ascertained what happened,” adding: “The reason we have so many witnesses is because we don’t know what happened. So we have to look at everything.”

She told jurors this meant it was even more important they tried to “winkle out” what happened.

Brighton and Sussex University Hospitals NHS Trust immediately removed Flash from the Royal Sussex after Mrs Blaber died.

The coroner previously expressed concern when the trust re-introduced the substance in March following complaints over cleanliness.

In a report in May, the Care Quality Commission (CQC) told the trust it must do more to protect patients from potentially hazardous cleaning materials and there was “no legitimate reason to decant cleaning products into other containers”.

Mrs Blaber, who was born in Tidworth on the Wiltshire/Hampshire border and lived in Lewes, East Sussex, was taken to the hospital’s accident and emergency department and admitted with a suspected minor stroke on August 22.

She had been housebound since the beginning of the year after becoming very frail with “significant and enduring medical conditions” including arthritis, chronic leg ulcers and had been treated for cancer, the inquest heard.

The widow, a retired shopkeeper, did not have dementia but was an “extremely vulnerable woman” who was “completely dependent” on the hospital for care, Mrs Hamilton-Deeley told jurors.

CCTV footage due to be submitted as evidence during the inquest appears to show cleaners removing cleaning fluid from storage and putting it on a trolley, the hearing was told.

CQC inspectors visited several of the hospital’s wards unannounced in October and expressed concern cleaning trolleys with hazardous substances were found unattended during the inspection, a report said.

But staff had good knowledge of rules for keeping cleaning products after being recently being retrained, the watchdog added.

The coroner tasked jurors with considering safety procedures put in place by the trust at the time and said the types of drinking jugs issued to patients on the ward would be of particular focus.

Mrs Blaber’s son Gary and his wife Sandra, who also live in Lewes, as well as her sister Rosemary Bird, of Eastbourne, all attended the hearing and described her as someone who was “very house proud, loved her home” with a good sense of humour despite her heath difficulties, the coroner said.

A post mortem found she died of respiratory failure with pneumonia being a contributing factor after ingesting cleaning fluid.

Mrs Hamilton-Deeley said it had been “erroneously reported” the liquid was bleach but it was not.

She added: “We are bound to call it by its name. It is Flash. There are other cleaning fluids. This Flash is not particularly noxious.”

In May Sussex Police said its investigation had been completed, there was no evidence of a crime being committed and there were no further lines of enquiry.

But at a pre-inquest review in June, coroner Veronica Hamilton-Deeley said a “deliberate act” could not be ruled out.

The trust previously said a “comprehensive review” of potentially hazardous substances took place immediately after Mrs Blaber’s death with improvements made to staff training and the way substances were stored and used.

The inquest continues on Monday afternoon and could last up to eight days.