The first public inquiry into mental health to be held in England is investigating the deaths of around 2000 people, with the number rising from 1500 following new information.
It comes as the chairwoman described the number of responses to the inquiry from current and former staff as "hugely disappointing".
Dr Geraldine Strathdee, chairwoman of the Essex Mental Health Independent Inquiry, said the initial figure of 1500 deaths was based on information from Essex Partnership University Trust (EPUT) and announced in March 2022. All of the 1500 died while they were a patient on a mental health ward in Essex, or within 3 months of being discharged, between 2000 and 2020.
In an open letter, she wrote: "In December 2022, I received an update on this number from EPUT, and it actually stands closer to 2000.
"This is a significant increase in the number of people who have lost their lives as mental health patients – and the number of families who have suffered this grief.
"I am concerned that it has taken 2 years since this inquiry was announced to be informed about these individuals' deaths by the Trust.
"My inquiry will now begin the process of writing to these families to offer to meet with all of them who would like to provide evidence to the inquiry."
Inquiry: Number of Responses From Staff 'Hugely Disappointing'
She said the inquiry has heard "remarkable and sobering evidence from many families" in the past year. But, she said the number of responses from current and former staff is "hugely disappointing".
She wrote: "Of the over 14,000 staff written to, we have received a small number of written comments from staff and, to date, only 11 have said they would attend an evidence session.
"Where we have also written directly to some of those involved in the cases of deceased patients we are investigating, 1 in 4 have responded to say they will provide evidence."
She said this is "inadequate to meet our terms of reference".
Dr Strathdee said "in the event that staff engagement remains very poor, it is my view that the inquiry will not be able to meet its terms of reference with a non-statutory status".
She said she met Health Secretary Steve Barclay on Tuesday to share her "concerns about the inquiry". "Any decision on the inquiry’s status will be the Government’s to make," she said. "I am confident that my concerns are being treated seriously.
"I will update my witnesses as soon as I’m in a position to do so," adding that the delay will delay publication of the inquiry's report. "These challenges will cause some time delay to the inquiry’s work, and this means we will not be publishing in spring 2023.
"I will provide an updated timeline as soon as I am able," Dr Strathdee said. "The inquiry’s work will continue while we await further progress.
"I remain committed to getting answers for families and patients as soon as possible."
Marjorie Wallace, chief executive of mental health charity SANE, said any delays could undermine the inquiry. She said: "What hope is there to improve care for the most vulnerable if the truth of the serial failures comes too late or not at all.
"It is not only an injustice to the families of those who have already taken their own lives but loses the chance of learning lessons to prevent tragedies in the future."
The inquiry was announced by Nadine Dorries in 2020, when she was a health minister, after a series of deaths on an NHS mental health unit in Essex. Dr Strathdee said the inquiry started to gather evidence from families of those who have died, as well as former patients, in December 2021. The inquiry also wants to speak to current and former staff.
Speaking last March, Dr Strathdee said that following the inquiry she will be "making recommendations to the Government on what changes must be made to keep patients safe in mental health inpatient care and to improve the experiences of their families and loved ones".
This article contains information from PA Media.