More than two thirds of people in the north west with illness or disabilities, who took the government to court after losing out on benefits, went on to win their case.
Nationally, more than half a million people were wrongly denied disability benefits from 2013 to 2018, an investigation found.
Charities and advice groups said the figures show the Government's benefits tests were fraught with "poor decision making" and "obvious inaccuracies".
Most appeals concerned Employment Support Allowance (ESA), which is paid to people who cannot work because of illness of disability; the Disability Living allowance (DLA), which is given to those with extra care or mobility needs; and Personal Independence Payments (PIP), which was brought in to replace DLA.
"The reason for the high success rates [at tribunals]...is because of the poor assessments carried out by health professionals," Daphne Hall, the vice-chairman of the National Association of Welfare Rights Advisers, said.
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"The DWP [Department of Work and Pensions] tend to base their decision purely on these assessments and disregard other evidence sent in by the claimant."
Initial assessments are carried out for the DWP by private companies Capita, and the Independent Assessment Services, formerly called Atos, which have come under fire for scoring claimants too harshly.
'DWP evidence would be wholly inadmissible in any other court'
Two years ago, Britain's more senior tribunal judge, Sir Ernest Ryder, senior president of tribunals, also said the quality of evidence provided by the DWP was so poor it would be "wholly inadmissible" in any other court".
Capita and Maximus, which also carries out medical assessments, said the majority of people were satisfied with the process, and they were working with charities and disabled people's organisations to improve their services further.
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The Government said appeals represent a small percentage of all benefits claims it handles.
The number of appeals heart at tribunal dropped 72 per cent from 2013/14 to 2014/15, but has risen steadily since.
'Huge drop in appeals down to controversial cuts to legal aid'
That initial reduction was down to the introduction of written reviews, so-called mandatory reconsiderations, experts said, which is the first stage of the appeals process when claimants ask the DWP to look at its decision again.
Cuts to legal aid were partly responsible for the drop in appeals, charities said. Prior to 2013, lawyers could receive funding for legal help, including advice, assistance, and casework.
At the DWP's Liverpool processing centre, which covers the north west, the success rate for disability, sickness, and incapacity benefits appeals at tribunal has risen from 37 per cent to 69 per cent over the five-year period.
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Overall, some 142,838 people who lost disability, sickness, and incapacity benefits appealed against their decision at tribunal, with 70,455 succeeding.
When all types of benefits are considered, the success rate rose from 32 per cent in 2013/14 to 66 per cent in period from April to December 2018, with 78,399 people successful.
'This process can be stressful and often demeaning'
But Emma Carrington, advice and information manager at Rethink Mental Illness, said the charity has "heard from countless people living with mental illness", who said appealing is "long and extremely bureaucratic."
She said: "This process can be stressful and often demeaning, and many people don't have the energy to challenge the decision. It takes a significant amount of time and energy to challenge an incorrect decision, so it's undoubtedly possible that some verdicts aren't taken to tribunal.
"These statistics demonstrate an assessment system that is not appropriately taking into account the needs of people living with severe mental illness, something that must change if the system is to improve."
'Without support, it's easy to get lost in the system'
Polly Neate, chief executive at Shelter, the housing and anti-homelessness charity, said "it seems obvious that the removal of legal advice in welfare benefit matters has led to a steep drop in cases being brought to tribunal, as people are no longer able to obtain legal advice and obtain representation for the early stages of the appeals process".
She added: "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.
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"Often people in these situations are already under a lot of stress and struggling to get by. Without support, it's easy to get lost in the system.
"Early advice is less costly and more efficient, and can fix problems before they are allowed to snowball. It's very frustrating when we find ourselves helping people in court in situations that could have been resolved much earlier without the need for stressful and costly court proceedings."
'The Government's relentless cuts have left the most vulnerable without essential legal aid and support'
The Government previously announced a long-awaited review of the legal aid cuts, which came as part of the Legal Aid, Sentencing, and Punishment of Offenders Act 2012 (LASPO).
Rachel Logan,, law and human rights programme director at Amnesty International UK, described legal aid as "the bedrock of a society where everyone is able to secure their rights and challenge injustice".
She said: "Amnesty's own research before the LASPO review was announced made clear that the Government's relentless cuts have left thousands of society's most vulnerable people without essential legal advice and support, often denying them the rights they are entitled to.
"Although this data is new to us, it would be no surprise that access to justice for those without deep pockets remains out of reach, or that LASPO continues to have a disproportionate impact on those most in need.
"There needs to be an urgent change of direction if we are to avoid slipping further into a two-tier justice system that is causing an untold amount of harm to some of society's most vulnerable people."
'We have been improving the assessment process'
The DWP refuted suggestions that are targets to upholding a decision made on a claim, and said it expects the "highest of standards" from the firms paid to carry out assessments.
They are done by trained health professionals with the "right clinical experience", while PIP was brought in to ensure mental health conditions "were given the same parity as physical conditions", with 64 per cent of mentally ill claimants now on the enhanced rate of daily living, compared to 22 per cent under DLA in May 2013, the department added.
A spokesman said: "We are committed to ensuring people get the support they are entitled to and spend £55 billion a year supporting disabled people and those with health conditions.
"We have been improving the assessment process and just four per cent of all ESA decisions and five per cent of all PIP decisions are overturned at appeal."
The Ministry of Justice said it would be "pure speculation to suggest a decline in the number of benefit appeals is solely as a result of changes to legal aid, given there have been many changes to the benefits system since 2013".
A spokesman said: "We recently announced a series of pilots offering support to people with social welfare problems, including early legal advice to try to solve issues before they reach court. Legal aid is available for onward appeals to the Upper Tribunal, Court of Appeal, and Supreme Court."
'We follow Government legislation'
A Capita spokesman said: "Our assessors achieve a consistently strong independent customer satisfaction rating from people going through the PIP process, with more than 95 per cent of claimants saying they are satisfied.
“We are committed to improving the PIP process and have invested in new ways of working to ensure it is as straightforward for people as possible, introducing a new system to allocate more convenient appointments.
“We follow Government legislation for all of our assessments, and work closely with charities and support groups to deepen our understanding of particular conditions.”
And a spokesman for the Centre for Health and Disability Assessments (Maximus) added: "Our role is to deliver high quality, sensitive and accurate functional assessments.
"All assessments are completed by registered healthcare professionals with clinical experience. We have consistently increased satisfaction levels and work in partnership with charities and disabled people’s organisations to improve the service further.
"Decisions on an individual’s benefit eligibility are taken by DWP and there is rightly an appeal process for those unhappy with a decision.”