Seven in 10 people overturned a Department for Work and Pensions decision to deny them disability support, figures have shown.
In the last three years, around 293,000 people won their cases after taking a decision to deny them Personal Independence Payments to a tribunal.
Around 5% of all assessments were overturned say the DWP.
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However, in the same period, some 500 people died before their initial challenge was decided.
In 2019, an investigation into the treatment of six-stone Stephen Smith, from Liverpool, - who was wrongly denied benefits before his death - found that the DWP 'followed policy'.
The 64-year-old was repeatedly and incorrectly turned down for benefits while suffering with a number of serious illnesses before his death.
Initial benefits assessments are carried out on behalf of the DWP by private contractors AT Capita, the Independent Assessment Services (formerly called Atos) and Maximus, reports the Mirror.
Any appeals must be filed with a month by the claimant before it can be escalated to a tribunal.
The DWP said most decisions were overturned on appeal because new evidence was submitted.
Former DWP senior mental health policy adviser Tom Pollard said: "Narrow questions and a sense of being under suspicion [during assessments] can make it hard for people to give a full account of the barriers they face."
The DWP said it supported "millions of people a year" and "the vast majority of PIP cases were not appealed".
PIP is a benefit meant to help people with a long-term physical or mental health condition or disability.
Why are claimants going to court?
The issue stems from a High Court ruling in December 2017 that found the DWP had not correctly followed regulations for the points given for being able to go on journeys.
Claimants should have been awarded the standard mobility rate of PIP if their psychological distress meant they were unable to undertake any unfamiliar journey without having someone with them.
And they should have been given the enhanced rate if they could not go on any familiar journey without support.
But instead the DWP had been underscoring people for this part of the assessment, meaning some didn't qualify for the mobility rate of PIP at all.
Some qualified only for the daily living rate - and if they weren't eligible for that either, they received no PIP whatsoever.
The department is now reassessing claims and making adjustments that will mean extra cash and a big back payment for many who were turned down the first time around.
Earlier this year, analysis by charity Benefits and Work found the department had paid out £18million to people who lost out because of this legal error.
One man received £12,000 backpay when the DWP finally agreed to review his case.
The claimant, who has a severe mental health condition, felt he should be eligible for a back payment of PIP.
After a long battle, including two requests to review his case and an attempt to take the DWP to a tribunal, officials agreed to carry out a 'mandatory reconsideration' and awarded him £12,000 in backpay.
The social security advice site Benefits and Work has identified a list of conditions that DWP documents say are most likely to be eligible for backpay.
The key conditions are:
Agoraphobia
Alcohol misuse
Anxiety and depressive disorders (mixed)
Anxiety disorders
Autism
Bipolar affective disorder (hypomania/mania)
Cognitive disorders
Dementia
Depressive disorder
Drug misuse
Learning disability
Mood disorders
Obsessive compulsive disorder (OCD)
Panic disorder
Personality disorder
Phobias
Post-traumatic stress disorder (PTSD)
Psychotic disorders
Schizoaffective disorder
Schizophrenia
Stress reaction disorders
Benefits and Work said: "It should be stressed that this was just the DWP’s prediction of who is most likely to be eligible. Just because your condition is not listed here, that does not mean that you are not entitled to a back payment.
"Nor, if your condition is listed here does it mean that you are going to get an award. It is clear that the DWP has chosen to make very few awards indeed."
How much is PIP support?
PIP is made up of two parts - a daily living rate for those who struggle with everyday tasks, and a mobility rate for those who need help going out or moving around.
The weekly rate for the mobility part of PIP is either £23.70 or £62.55, equivalent to £94.80 or £250.22 a month.
