HM Revenue and Customs (HMRC) pursued John Clark of Dunfermline, who has the mental development of a 12-year-old, for £18,000 in unpaid tax.

But First-tier Tax Tribunal judge Kenneth Mure has ordered the bill be cancelled and excoriated HMRC for failing to make any concession for Mr Clark’s vulnerabilities.

Mr Clark, who worked part-time as a painter and decorator, was able to deal with his paperwork until he split from his wife in 2003.

She died ten years later and Mr Clark became sole carer of their daughter.

The family’s home was destroyed by a fire in 2006 which meant the daughter could not go back there for nearly a year.

However, HMRC demanded £17,779 covering six years, during which time he was accused of failing to declare his earnings.

A plea his daughter sent to HMRC was returned unopened, marked ‘sent to wrong department’.

Mr Mure called the tax authorities dogged pursuit of Mr Clark “unconscionable”.

He said it was “completely unreasonable, outrageous, grossly unfair and without regard to what is right”.

The judge added that HMRC’s treatment of Mr Clark was “too narrow, inadequate and lacking in any consideration of his peculiar vulnerability”.

He added: “It ignores his inability to engage fully and satisfactorily with the tax authorities. It neither recognises nor makes any concessions to his vulnerability.”

HMRC was ordered to grant Mr Clark “special relief” and cancel his outstanding tax bill.

A spokesman for HMRC said: “We aim to treat all our customers with fairness, respect and compassion, but we clearly got it very wrong in this case and badly let down a vulnerable customer whom we should have helped.

“We apologise unreservedly to Mr Clark for the worry and stress we caused him. We will investigate what happened in Mr Clark’s case, to ensure that this doesn’t happen again.”