One in two people who appealed in court against a decision to deny them disability benefits was successful, analysis of five years of data shows.
In total, more than 550,000 people won an appeal over their benefits at tribunal between 2013 and 2018.
Ann Barker, who has bipolar disorder, said she was tempted to give up, but had twice fought a decision at tribunal and won.
The government said only around 5% of disability decisions were overturned.
Benefits assessments are carried out on behalf of the Department for Work and Pensions (DWP) by the private contractors Capita, the Independent Assessment Services (formerly called Atos) and Maximus.
The success rates showed benefits assessments were beset by "poor decision-making" and "obvious inaccuracies", charities said.
In 2018, the Commons Work and Pensions Committee said failings in disability benefits assessments had led to a "pervasive lack of trust" in the system. It said ministers should consider taking the process back in-house.
Daphne Hall, the vice chair of the National Association of Welfare Rights Advisers, said: "The reason for the high success rates [at tribunals] is because of the poor assessments carried out by health professionals.
"The DWP tend to base their decision purely on these assessments and disregard other evidence sent in by the claimant.
"However, tribunals will weigh up all the available evidence and talk to the claimant further, which enables them to make much more reasoned and balanced decisions."
The BBC's Shared Data Unit analysed figures from Freedom of Information responses from HM Courts and Tribunals Service and Northern Ireland's Department for Communities (DfC).
Most of the appeals concerned Employment Support Allowance (ESA), which is paid to people unable to work because of illness or disability; the Disability Living Allowance (DLA), which is paid to people with extra care or mobility needs; and Personal Independence Payment (PIP), which was introduced to replace DLA
- About 553,000 successful appeals were heard at tribunal about disability, sickness and incapacity benefits out of 981,000 from 2013 to 2018
- Last year, around two thirds of cases heard in Great Britain found in favour of the claimant. In Northern Ireland, the figure was around 54% in 2018-19
What is the appeal process?
Since 2013, people seeking to overturn a benefits ruling must complete a written challenge within a month, known as a mandatory reconsideration. If unsuccessful, people can appeal against the decision at tribunal.
The DWP said mandatory reconsiderations were introduced to ensure claimants received the right decision without having to go to court.
Critics say the process is confusing, stressful and does not give claimants enough time to gather evidence to support their appeal.
'I was down to my last tin of food'
Stephen Naish, 60, from Poole in Dorset, was a sheet metal worker for 16 years but was forced to quit work following a motorcycle accident that left him paralysed in his right arm.
He now has curvature of the spine, a frozen shoulder, carpal tunnel syndrome, and damaged tendons in his left arm.
Last year, his Employment Support Allowance was removed following a health assessment.
"My income was something like £30 to £40 a month less than my outgoings, without my travel and food," Mr Naish said. "I was literally down to my last tin of potatoes.
"I had to go to a food bank and I was very lucky as I borrowed money from my wonderful mother and a good friend. It was stressful."
The government declined to overturn the decision following his written appeal, but he won at tribunal in May.
Judges heard he was unable to pick up a £1 coin with either hand.
'I kept wanting to give up'
Ann Barker, from Norwich, who has bipolar disorder, psoriasis, arthritis and lower back pain, has twice taken the government to tribunal and won.
In 2013, she appealed against a decision to refuse her the mobility and care components of Disability Living Allowance.
Then she challenged a second time when she was refused an application for a Personal Independence Payment.
Ms Barker said: "I kept wanting to give up, saying to myself I don't want the money, I will manage.
"They were asking me if I could find a place of work and I was saying on a bad day [when she is depressed], I can't leave my bedroom.
"They have a protocol but it's all geared towards physical health disabilities. The last tribunal obviously heard me, they knew I had hidden disabilities."
Emma Carrington, advice and information manager at Rethink Mental Illness, said: "We've heard from countless people living with mental illness that challenging welfare decisions is long and extremely bureaucratic.
"This process can be stressful and often demeaning, and many people don't have the energy to challenge the decision.
"Many assessors don't understand the fluctuating nature of mental illness."
The DWP said the health assessments were carried out by professionals with the "right clinical experience".
Drop in appeals
The rise in the percentage of successful appeals came despite a drop in the overall number of cases being heard at court.
Charities said the introduction of mandatory reconsiderations and cuts to legal aid had deterred people from appealing.
"If someone is already struggling to navigate the labyrinthine benefits system, then they are probably going to struggle to navigate the courts system without help," said Polly Neate, chief executive at anti-homelessness charity Shelter.
Kamran Mallick, the chief executive of the Disability Rights UK charity, said he had not been able to walk since the age of three but was asked to try and stand up at his own PIP assessment.
He said: "I was asked if I could try standing up. That's just crazy, because I can't. And yet despite that, I was asked to try harder.
"It's kind of dehumanising, degrading. It feels like you are being told that you are not telling the truth. "
The Ministry of Justice said it was "pure speculation" to suggest a decline in the number of benefit appeals was down to legal aid changes.
A spokesman said it would pilot a scheme next year offering early legal advice to people with social welfare problems.
The DWP said it had been improving the assessment process.
It said of 3.3 million PIP decisions taken from 2013 to 2019, only 5% were overturned at appeal, while of 4.4 million ESA decisions made between 2014 and 2019, only 4% were overturned at appeal.
A spokesman said: "We are committed to ensuring people get the support they are entitled to and spend £55bn a year supporting disabled people and those with health conditions."
The Department for Communities in Northern Ireland said about 10% of all of the PIP decisions it handled were appealed, with about 2.5% of the overall number of cases successful.
Capita and Maximus said the majority of people were satisfied with the process and they were working with charities and disability organisations to improve their services further.
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