Francis Bacon died in April 1992, to the very day six months short of his 83rd birthday, and Tate Britain is holding a centenary retrospective exhibition of his inseparable art and life.

Long in the planning, the show is to be shared with the cities of Madrid, where he died of the cumulative effects of pneumonia, asthma and a heart attack, and New York, where he had his most profitable success.

Inseparable art and life? For most of his many years it was simply not possible to speak or write of Bacon's private life, whispered to be not merely disreputable but punishable, at least in his first 50 years, by sanctions almost as harsh as those for treason and murder.

Though to critics of any sensibility it was quite obvious that this private life was largely the source of the imagery and energy in his paintings and unquestionably crucial to his aesthetic development, there were others who —through overwhelming prominence on the Arts Council and on television, almost as celebrated as himself for their performances as his interpreters — gave us a Bacon distorted and bowdlerised.

In their constructs he could discern little of himself but in a sense he was content with their dissembling, for it kept him camouflaged and his private life remained largely private to the end.

Though he knew them to be in error, his conviction was that in time their interpretations would be recognised as fraudulent, then discarded, letting his paintings speak for themselves.

In his words: “Painting is its own language and is not translatable into words''.

I first encountered Bacon's paintings when I was a schoolboy and am convinced that at the Hanover Gallery, Erica Brausen, his first dealer until 1958, showed paintings that I have not seen since; this is true, too, of an exhibition at the old Beaux-Arts Gallery, where Helen Lessore had his paintings in 1953.

It is therefore with great interest that I await the publication of a catalogue raisonné. In London in the 1950s it was impossible not to be aware of Bacon.

After his transference to the Marlborough Gallery and the Tate's first retrospective of his work in 1962, it seemed to me from then on that no matter where I went in Europe and America I ran into more or less the same travelling Bacon circus — in Chicago and New York, in Turin, Kassel, Mannheim, Zurich.

It became increasingly evident that the formerly slow-thinking and slow-painting painter, in abandoning the considered, deliberate and frequently revised terribilità of the early works, was at risk of becoming slick and habitual, even intellectually easier and emotionally shallower, and that the output of his pictures of ambitious scale was mightily increased, raised to some 20 canvases a year instead of two or three.

It was at this point of sudden but shrewdly engineered success that I first encountered Bacon. I was to know him for 30 years or so.

Our acquaintance developed from an enquiry I had to make when a painting said to be by him was delivered to Christie's and I doubted it.

Bacon was not then the sort of painter whose work Christie's liked to sell but was nevertheless one whose work I thought they ought to sell — though not if it was a forgery.

I knew that in Milan a forger was producing, even at that early stage, almost plausible pastiches.

As Bacon answered neither telephone nor letter, I risked knocking on his door on my way home, was kindly, if briefly, received, and the picture's authenticity denied.

Milanese forgeries again came into play in the later 1960s (a small London dealer was importing them, their quality menacingly improved) and again I had reason to see Bacon, the acquaintance cementing to the point where Bacon felt that he could, for example (since I lived only half a mile away), telephone at crack of dawn and ask me to drive his friend, John Edwards, disablingly hungover, to a family conclave in Long Melford.

For me the most fruitful period of our relationship began when Harrods opened a juice bar in a corner of the food department. Thither went Francis almost every morning and if our paths crossed, I joined him for breakfast.

All this was long before I began to earn my living as a critic and I neither took notes nor made recordings to publish as a precious interview.

I formed the opinion that we spoke of two honest Bacons, with an unmentionable commercial third Bacon waiting in the wings; the first was the kaleidoscopic, fragmentary Bacon, wit, gossip, gambler, drinker, traveller, willing supporter of such unlikely young painters as Anthony Zych and Michael Leventis, social performer and frivolous lost soul and, in strong contrast, Bacon the found soul, the melancholy painter, utterly intense, the one a relief from the other, though the onlooker could never quite tell which of these lives he found the more unbearable.

The third Bacon was the painter preparing for the next commercial exhibition, the repetitious Bacon, the Bacon who had done it all before, the idea and image stale, the clashing fields of colour too much assured with practice, the drawing and construction occasionally so casual as to deprive the painting of any intended significance.

The third Bacon resorted to tricks and cyphers without meaning in the early 1980s to flat arrow-heads in black or white or red that seem to act as jarring indicators (but of what?), and in the late 1960s to splashes of dense white paint strung across the entire surface.

Bacon had no formal training as a painter and for some time worked as an interior decorator, maker of furniture and, occasionally, as a gentleman's gentleman.

Little is known of his early paintings, rejected by English Surrealists (whom he would otherwise have joined) as not sufficiently surreal, and he destroyed most of them before exhibiting, in 1945, Three Studies for Figures at the Base of a Crucifixion, one of the disappointingly few works in the permanent collection of Tate Britain.

With this, a work as important to Bacon as Les Demoiselles d'Avignon was to Picasso, he emerged as a painter of pessimistic imagination, his images coldly barbaric, in full command of the techniques of oil painting, to assault the nervous system of the orthodox and frivolous art world of postwar Europe.

Orthodoxy capitulated and Bacon became the most exhibited of British painters. Successive directors of the Tate described him as Europe's or Britain's greatest living painter, though none in America or any other continent was greater.

Bacon continued the line of ancestral European painting, the descent from the grandeurs of the High Italian Renaissance and the bloodstained violence of its German equivalents — two years before his death he went again to Colmar to Grünewald's Isenheim altarpiece — when all about him the aesthetic nonsense of abstraction and a host of pretentious transitory fashions were the norm.

He took the Crucifixion, stripped it of all its Christian implications and invested it instead with the universal beastliness of man and abattoir, running with blood.

He took the mouth and used it exclusively as a feature of violent expression, deafening us with its screams.

He took the portrait and, refusing to chart features or delve into character, became the harsh interrogator provoking the betrayal of body language, the man outside the ring of light, the man with the lash and cigarette butt.

His prisoners, presidents popes and lovers, squirm. All are, in a sense, himself. To women, however, always on a smaller scale, he applied only the torments of his style.

He was capable of an extraordinary fusion of intellectual and painterly devices that are spatial, flat, abstract and narrative, the logic of their complexities never failing even at the end — not for Bacon the empirical incompetence of Picasso's dotage, the last years as an idiot in the antechamber of death.

He used the image of the trap, the cage, the cell, the X-ray field and the heavy fall of light from the single naked bulb to imprison and torment his subjects and to assault complacent senses with graceless nakedness and vomit in the washbasin.

His insistence that his pictures be protected with plate glass deliberately added a disturbing layer of illusion when the visitor inevitably found his own reflection between himself and the subject within, seeming to play some part in the sordid drama, spectator become participant.

Bacon and his images were nourished by his extensive knowledge of paintings by old masters, Cézanne, Degas and Picasso, by his interest in the subconscious development of images, his enquiry into the quasi-supernatural field of the emanations, auras and energies of his subjects, by his interest in crime, violence and disease, by his collection of the horrible in medical publications.

He took the vile, the visceral, the politically obscene and so lifted them with paint that we can contemplate ferociously profane images of torment, cruelty and despair and perceive in them an inheritance from the great Renaissance themes of religious and temporal power, the classical pantheon of ancient deities, the Christian pantheon of martyrs.

Titian, Rembrandt and Velazquez, were they to beg entry to this latest exhibition, might not care for Bacon's personal pantheon.

But I have no doubt that they would recognise kinship in his mastery of paint and the pessimism of his images.

As an atheist and as an artist for whom money was at least as important as the message of his work, he was the perfect mirror of the spirit of his age.

A Francis Bacon retrospective is on at the Tate Britain, London, until January 4, 2009.