Relatives of a man who starved to death after his benefits were cut are taking legal action against the Department of Work and Pensions (DWP).
Errol Graham's family said they hoped the case would overhaul the system "to better protect vulnerable claimants".
The 57-year-old's body was found weighing four-and-a-half stone (30kg) in June 2018 by bailiffs who broke into his Nottingham flat to evict him.
The DWP previously said it was still "reviewing its safeguarding processes".
An inquest into Mr Graham's death in 2018 heard his benefits had been cut in October 2017.
His daughter-in-law Alison Turner has now sent a pre-action protocol letter to the DWP arguing the termination of benefits for someone in Mr Graham's circumstances were unlawful.
She also argues secretive investigations and reviews being conducted by the DWP into benefit-related deaths are unlawful and must be reformed.
"They [DWP] have been so deceitful and they think they can carry on and not answer for it," said Ms Turner.
"Enough is enough.
"They need to accept a duty of care. They let Errol down, they failed him by not gathering more information on him before deciding to cut his benefits."
Ms Turner released details of an unsent handwritten letter she found in Mr Graham's flat in which he pleaded for authorities to "judge me fairly".
"Coping with this lifestyle wears me out," he wrote.
"Being locked away in my flat I feel I don't have to face anyone but at the same time it drives me insane.
"I feel more secure on my own and in my own company but I wish it wasn't like that.
"All I want in life is to live normally, that would be the answer to my prayers."
Mr Graham's family were not told his benefits had been stopped and said he broke off contact with them due to his mental health.
The inquest heard he had a history of depression, missed GP appointments, refused help from mental health teams and ignored contact from the DWP.
At the time, Coroner Elizabeth Didcock asked for "robust policy and guidance for DWP staff" to prevent future deaths.
"The government owes it to Errol, his family and the country to explain why the DWP has repeatedly failed to learn from these tragedies over many years," added Ms Turner.
"In Errol's memory I am determined to fight for change so that no more families have to live through the horror we have."
A DWP spokesman said: "Our sympathies are with Mr Graham's family. It would be inappropriate to comment further at this time."