A former top civil servant has criticised the disability benefits assessment system as a “hostile environment” after being told he was ineligible for support despite having Parkinson’s and terminal prostate cancer.

Andrew McDonald, 56, who ran the parliamentary body overseeing MPs expenses before retiring on health grounds, had his benefits stopped after assessors decided he was no longer ill enough to qualify for personal independence payment (PIP).

McDonald described the assessment process undergone by hundreds of thousands of chronically ill and disabled people each year as crude, unprofessional and Kafka-esque in its complexity.

“I was shocked by the way this was being administered against the interests of some of the most disadvantaged people in the country,” he said. “PIP is beset by profound administrative failures which work to the disadvantage of disabled people.

“My personal interactions with the [PIP] process were perfectly pleasant; but the system as a whole does create the impression of it being a hostile environment and one where two of the foes are complexity and the sense that it is not a level playing field [for claimants]”.

McDonald, who chairs the disability charity Scope, said: “I thought this was a system to give people a hand up; in practice they encounter a sleight of hand that is completely out of kilter with the best traditions of British public service in which I was not only raised but worked for most of my career.”

He called on the government to look afresh at the entire PIP process, which has been dogged by controversy since it was introduced in 2014 as a way of cutting the disability benefits bill. The controversial assessments are carried out by private contractors Atos and Capita.

Earlier this year, a cross-party committee of MPs concluded that the PIP process was error-strewn and insensitive, after hearing evidence of poor practice from more than 4,000 claimants. The government responded that it was working to improve the system.

McDonald, who has challenged the decision to stop his benefits, said it was shocking that the latest official figures show 71% of PIP decisions were overturned when appealed against at tribunal. “The government should set aside any notion that PIP has been fixed,” he said.

PIP is intended to help with the additional costs of living with disability, estimated at £570 a month. It is not means tested or related to employment status and is typically used to meet special transport needs or health-related heating or food bills.

McDonald was diagnosed with Parkinson’s, a degenerative disorder of the central nervous system, in 2007. Three years later, he discovered he had prostate cancer and was subsequently told by doctors it was incurable. He is shortly to undergo brain surgery in attempts to mitigate some of the worst effects.

He retired in 2014 as the chief executive of the Independent Parliamentary Standards Authority and qualified for PIP the following year, a decision which was reaffirmed at review in 2017. Each time, assessors awarded him 11 points, comfortably over the eight point threshold for lower-level PIP support, which amounted in his case to about £3,000 a year.

At his third review assessment in March, however, he was awarded two points – a decision which suggested his health was improving. “I was flabbergasted: I had two progressively degenerative conditions and my Parkinson’s had become worse since the turn of the year,” said McDonald. “It’s now described by my neurologist as ‘very severe’.”

The assessor – an occupational therapist – had little understanding of Parkinson’s, he said. “She did a common test of twisting the forearm at the elbow. She concluded from this I have full power in my upper limbs. It’s nonsense. Daily, it is a nuisance to me that I am weak as a kitten in my upper body. ”

The system failed to account for the way Parkinson’s fluctuates dramatically, through the day and over time, he said. “I got dressed today without too much trouble, though my cufflinks eluded me. Two days ago, it took me 90 minutes because of the tremor in my hands, my lack of strength. Putting on jackets is a nightmare.

“That sort of variation from day to day could be captured in the system but, in practice, the people in the system I was working with were not capturing it.”

He asked the Department of Work and Pensions (DWP) to review the decision but it upheld it on the grounds that, although it was “surprising”, the assessment showed his condition had improved and it was up to him to supply evidence to prove that it had not. The DWP was effectively suggesting the loss of his benefit was his own fault, he said.

A DWP spokesperson said it monitored the quality of the PIP process to ensure it worked well for everyone. “We constantly seek to improve the quality of PIP assessments. We have undertaken two independent reviews of PIP and most recently announced that we will pilot video recording of assessments, to help increase people’s trust in the assessment process.

McDonald’s benefits were stopped in April and he lodged a tribunal appeal against the decision within weeks. Four months on, he is still awaiting a date for a hearing.

Although the government has promised that people with long-term degenerative diseases should no longer be given annual PIP assessments, this does not apply to claimants, such as McDonald, who received lower-level awards.

He had been able to manage financially without PIP, he said, but he was aware not everyone could. “If you are on a low income the sudden decision to stop PIP is a really serious blow – and it’s a blow from a bewildering system.”