He was the subject of awards by the score, including the Jerwood Painting Prize, and his works regularly sold for up to £60,000. Yet, for many, his still lifes, landscapes, portraits and crucifixions remain a closed book.

Tate Britain has half-a-dozen Aitchisons in its collection, but when are they on display? To his devotees, Aitchison remains a figure of the utmost importance, both as a Colourist and a visionary in the mould of William Blake and Samuel Palmer. Two of his works, including The Crucifixion, hang in Cawdor Castle, Nairn, personally collected by the late 6th Earl Cawdor (1970-93), alongside works by Victoria Crowe, Simon Palmer, John Piper and a cartoon by Salvador Dali.

Aitchison conformed neither to school nor fashion but earned critical esteem and some popularity as an artistic loner, painting in a spare but powerfully evocative style that some say remained more or less unchanged for 50 years.

If visionaries are supposed to look eccentric, Aitchison rarely disappointed. White hair streaked with black above a baggy jumper, he reeked the same shapelessness and disorder that emanated from his London studio. Yet for his artistry, truth was of the first order, and any deviation from it anath­ema. So he made a name for himself painting Christ’s crucifixion, relating the event through his work, even including his beloved Bedlington terriers in some of them.

John Ronald Craigie Aitchison was born in Edinburgh, the son of Craigie Mason Aitchison, KC, first socialist lord advocate for Scotland. Son of a United Free minister, Craigie’s father reacted by taking his family to church services from

Baptist to Roman Catholic. The breadth of his religious upbringing, combined with a visit to Glasgow to see Dali’s Christ of St John of the Cross at Kelvingrove Gallery, created a lasting impression. His venture into painting crucifixions began because “… it [Christ’s Passion] is a horrific story, and more worth trying to say something about than anything else that’s happened since”.

Educated at Loretto and at home, Aitchison was spared wartime

service owing to a medical report, and studied law at Edinburgh University and in London, moving to the Middle Temple. His spare time he spent at the Tate, copying works of art. When he failed his law exams, he found refuge in 1952 in the Slade, where among his fellow students he made friends of Michael Andrews, Paula Rego, Myles Murphy and Euan Uglow – the last a friend for life.

To Aitchison, the Slade was

“para­dise”, and he was encouraged by L S Lowry, one of the visiting teachers (though, equally, he was put off by John Piper and Victor Passmore).

Winning a British Council scholarship in 1955 to travel to Italy soon after leaving the Slade, he and Murphy set off for their destination in an ancient London taxi, freewheeling down Alpine passes to save petrol. The journey changed his life, for, having never studied art history, he was quite unprepared for the impact of Renaissance paintings he discovered – and the impact of the Italian landscape.

The expedition changed his life by the simple expedient of being introduced to raw umber. “It is all over Italy, in the fields,” he recalled. “I had got into painting bright colours before I went there, but I discovered that dark colours could be just as beautiful” – and he balanced his raw umbers against muscular reds, brawny blues, powerful yellows and the gentleness of deep blues and vivid pinks. In 1975, he bought an isolated house in the Tuscan hills at Montecastelli, not far from Siena, and spent much of the rest of his life in his adopted country. A constant source of surprise to visitors was the contrast between the bohemian clutter of his London house and austerity of his home in Tuscany.

Never able to sever his roots, Aitchison frequently returned to Scotland until, in 1963, he moved permanently to London, creating a home in Kennington that he transformed in brilliant colours and cluttered with the bric-a-brac he loved.

He rarely looked back to Edinburgh after that, confessing to feelings of apprehension before the opening of an exhibition in his home city some eight years ago.

The discipline of his father’s legal background never left him, and into his 80s he would still rise early to paint. By 33, he had his first one-man show, at the Beaux Arts Gallery in London. His solid achievement was reflected at the other end of his life, when, as well as at the Royal Academy, a major retrospective of his work from 1953 was held at the Serpentine Gallery in 1981, with further retrospectives in Leeds in 1994, and in Glasgow at the Gallery of Modern Art in 1996. Six years ago, his diary included a solo show at the Royal Academy.

Aitchison maintained an otherworldly outlook. Told that Elton John had bought one his of paintings, Aitchison asked: “Who?”

Aitchison never married. He was appointed CBE in 1999.

Painter; Born January 13, 1926; Died December 21, 2009.