One of the world’s best-selling authors was virtually illiterate before joining the Army, an audience at Devizes Festival heard on Saturday evening.

Former SAS soldier Andy McNab held his audience spellbound at the Corn Exchange with a brief resume of his Army career, his capture in Iraq which formed the basis of his first book, Bravo Two Zero, and his life to date.

Famously abandoned in a carrier bag on the steps of Guy’s Hospital in London as a baby, Mr McNab – not his real name – was in and out of care all his childhood and adolescence and, almost inevitably, got into trouble.

While serving a sentence in a borstal – which nowadays would be termed a young offenders’ institution – he was given a lecture by an Army recruitment officer and, to coin a phrase, it changed his life.

As a 16-year-old recruit to the Infantry Junior Leaders Battalion, he underwent an education course that was, eventually, to lead to his illustrious literary career.

Starting with Janet and John primers, Mr McNab eventually graduated to “proper books” and his love of the English language.

After tours of Gibraltar and Northern Ireland, he applied for selection to the Special Air Service, the crack special ops regiment that was formed during World War Two.

This led to undercover work against active service units of the IRA in Northern Ireland which gave him great satisfaction.

But it was the first Gulf War of 1991 that was to cement his reputation. He led an SAS group seeking to disrupt the launching of Scud missiles into Israel, when it was compromised after encountering a young goatherd.

Three of the group were killed and four, including Mr McNab, captured, incarcerated in the notorious Abu Ghraib prison where he was tortured.

He and his colleagues were eventually exchanged for Iraqi prisoners of war and when he was approached to tell his story to an MOD publicity team, he insisted on writing it himself.

Bravo Two Zero is still selling millions of copies worldwide, along with the many other books, both fiction and non-fiction, that he has written.

He said: “In the Army we were told we were not stupid, just uneducated.

"We all had the literacy and numeracy levels of 11-year-olds but I am now writing 900-word pieces for the Daily Telegraph and other newspapers.”

After taking questions from the audience, Mr McNab signed copies of his books in the foyer. It was a very long queue.