Half of all people who appeal in court against disability benefit decisions win their cases, analysis of five years of data by the BBC has shown.
The BBC's Shared Data Unit found that over 550,000 people won an appeal over their benefits at tribunal between 2013 and 2018.
The government says that only around 5% of 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 that "hideous" benefits assessments were beset by "poor decision-making" and "obvious inaccuracies" according to research by the BBC.
Disability Rights UK's Chief Executive Kamran Mallick told the BBC that 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 Commons Work and Pensions Committee said last year that there is a "pervasive lack of trust" in the system and advised that Ministers should consider taking the process back in-house.
The full story and dataset produced by the BBC can be found here: https://www.bbc.co.uk/news/uk-49891159