Around 100 people denied fast-tracked disability benefits for terminal illness died challenging the decision, bbc.co.uk has highlighted.
Currently, patients can access benefits more quickly if their doctors say they could die within 6 months - for example, if they give them a life expectancy of 5-12 months.
In July the government said it will introduce a new 12-month end of life definition.
However, end-of-life charity Marie Curie said it had "serious concerns about the DWP's ability to recognise when a claimant was approaching the end of life and was in desperate need of support".
Dr Sam Royston, its director of policy and research, said if people died before their appeals were decided it was "highly likely they were living with a long-term or terminal illness".
"Many of these people should have received fast-track access to support [originally]," he added.
The Motor Neurone Disease (MND) Association said the unpredictability of the speed with which a person with MND might deteriorate, meant it was difficult for neurologists to give accurate prognoses.
Susie Rabin, head of policy and campaigns at the MND Association, said that made it difficult for people to access benefits through the special rules.
Dr Catherine Millington-Sanders, the clinical lead for palliative and end of life care for the Royal College of GPs, added doctors "still faced uncertainty when determining exactly how long someone has got to live".
"It makes little sense to have a rigidly set time cap to be a determining factor," she said.
Affected families and charities said the current process was "overly complex and stressful" and it remained unclear when reforms would happen.
They have also called for the government to go further and scrap the requirement for people on terminal illness benefits to be reassessed every three years.
Source: Hundred people die fighting for terminal illness benefits available from bbc.co.uk.
See also our related news story Scrap six months campaign victory: fast-tracked access to benefits for people with terminal illness expanded.