A campaign group's video of a patient dying in agony in a hospice has provoked outrage from doctors and nurses, who say it is '"scaremongering" and "sensationalist".
Dignity in Dying, which produced the film, said it was "a realistic portrayal" of the deaths of a minority of people in the UK.
In a report, it claims 17 people die in pain every day, despite the best care.
Hospice UK said the video was "designed to frighten vulnerable patients".
"This is misleading and irresponsible campaigning," said chief executive Tracey Bleakley.
"A sensationalist and inaccurate video" had been released to publicise Dignity in Dying's report, which was actually a "sensible report worthy of widespread discussion", she said.
But no campaign could justify scaring people away from the care and support they desperately needed, she added.
On Twitter, healthcare professionals lined up to criticise the film.
An NHS palliative care doctor said: "The very last thing the debate on assisted dying needs is deeply misleading and wildly inaccurate portrayals of dying in a hospice."
A palliative care nurse tweeted: "I have witnessed 100s of deaths. However, I have NEVER seen someone die like this."
"This is not truth," tweeted GP Dr Andrew Green, who has been a GP for 40 years.
40 years ago I started studying medicine, I've cared for people at all stages of life, I have never ever seen this in a hospital, hospice, or patient's home.— Andrew Green (@DrAndrewGreen) September 7, 2019
To scare people about what their end might be to further your aims is inexcusable.
This is not truth.
But other people recognised the portrayal of suffering in the film.
"I witnessed my wife suffering like this in a hospice. I think you need to accept that pain meds can't always alleviate the pain," said David Felton on Twitter.
I witnessed my wife suffering like this in a hospice. I think you need to accept that main meds cant always alleviate the pain.— David Felton (@Daveyji) September 8, 2019
"My mum died exactly like this, in spite of the commitment and dedication of the hospice drs and nurses," wrote another tweeter.
"She was in agony for 11 days. It was horrendous."
Dignity in Dying, which supports assisted dying, said palliative care could "provide a good death to a great many people", but for a small number it was not possible to relieve all their pain and suffering.
The group said this was reflected in its report, which contains graphic accounts from bereaved relatives who had witnessed a family member suffering towards the end of their life.
'Real world experiences'
"This film was designed to convey complex subject matter in an accessible way," it said.
"While the characters and storyline are fictional, they are a composite of some of the real world end-of-life experiences that people shared with us for our research.
"We took clinical advice throughout the creative process and are confident that the video is a realistic portrayal of what can and does happen to a small but significant proportion of dying people in the UK."
Dignity in Dying wants to see a change in the law to allow assisted dying as an option for terminally ill, mentally competent adults.
It argues this would improve the quality of life and death for many dying people in the UK.
Assisted dying is legal in The Netherlands, Canada, Australia and some parts of the US.