Rainier Louis Henri Maxence Bertrand Grimaldi was born on May 31 1923 into a princely family whose meagre possessions had never inhibited their capacity for vicious internal feuds. The Grimaldis first made their mark in the 12th century, as a seafaring (not to say piratical) family in Genoa. Their connection with Monaco dates from 1297, when François Grimaldi, known as "the Spiteful", captured the Rock from the Genoese, who had fortified it since 1215.

Although François was soon expelled, a kinsman of his bought the lordship of Monaco in 1338; and soon afterwards added to it the nearby villages of Roquebruna and Mentoneas (now Roquebrune and Menton). Although the Grimaldis were again chased off the Rock by the Genoese in 1355, they were able to re-purchase Monaco in 1419.

This time they clung on to their prize, though compelled in the early 17th century to accept Spanish protection. No one troubled to object when, in 1612, the lord of Monaco began to refer to himself as "Prince".

The Grimaldis lost their principality during the French Revolution, when Monaco became part of the Alpes Maritimes département of France. Even after it was restored to them in 1814, the Grimaldis hardly felt secure. Indeed, in 1861, after France and Sardinia had joined forces to throw the Austrians out of Italy, France gained Menton and Roquebrune, where the Grimaldis' taxation had long been resented. But Monaco itself emerged from these negotiations with its independence confirmed.

Meanwhile Prince Charles III, or more accurately his mother Princess Caroline, had hit upon the means of restoring the Grimaldis' fortunes by introducing a casino in 1857.

The Société des Basins de Mer was set up to run the enterprise, with the stipulation that no native Monégasque should be allowed into the casino. The business did not prosper until the injection of Rothschild money made possible the building of hotels, and of a new casino which opened in 1864.

So Monte Carlo (named after Prince Charles) was born. Yet, though the Grimaldis were now well off, they remained as quarrelsome as ever. And at the end of the 19th century, their natural talent for discord was sharpened by a startling mésalliance.

A laundrywoman who worked for Louis Grimaldi, Rainier's grandfather, asked him to keep an eye on her daughter Marie, who worked in a Montmartre night club, when he went to Paris. This Louis did to such effect that he had a daughter with Marie, called Charlotte.

For some time the rest of the Grimaldis would have nothing to do with either Marie or Charlotte. But in the absence of any other heir, Louis's father, Prince Albert, arranged in 1919 for Charlotte to be legitimised and (the next year) married to an impoverished French aristocrat, Pierre, Comte de Polignac. A daughter, Antoinette, was born at the end of 1920; and three years later Rainier became the first Grimaldi heir to be born in Monaco since 1758.

Having discharged his procreative duty, de Polignac, sensible of being disliked almost as much by his wife as by his father-in-law, left Monaco. "To make love," Princess Charlotte complained, "he needs to put a crown on his head." By the terms of his divorce settlement, he was forbidden to return to Monaco; and the carabineri were given instructions to eject him bodily if he flouted this ban.

The Grimaldis employed an English nanny, Kathleen Wanstall (said to be a cousin of Winston Churchill), who resolutely refused to speak, or even listen to, a word of French. De Polignac, who had retained rights over Rainier's education, insisted on his going to an English prep school, Summer Fields, at St Leonards, where he was known as "Fat Boy Monaco".

Rainier went on to Stowe, from which he ran away; when he was returned to the school the authorities placed him in the sanatorium. Meanwhile, Prince Louis and de Polignac continued to wrangle over their rights to control the boy until the British High Court decreed, in March 1936, that Rainier should be returned to Louis's custody. Thereafter the prince was sent to an easy-going school at Le Rosey, in Switzerland, where he was much happier.

He had just gone to Montpellier University when the Second World War broke out. After the fall of France, Prince Louis of Monaco threw in his lot with the Vichy regime, whereas Rainier, who remained in Montpellier, followed his father in developing sympathies with the Free French. He also acquired a mistress, the French actress Gisèle Pascal.

In 1943 he graduated from Montpellier and went on to the Ecole Libre des Sciences Politiques in Paris. He returned to Monaco in September 1944, a month after the Germans had left, and demanded (unsuccessfully) that the principality's pro-Vichy minister should be dismissed.

Rainier then joined the French army, and was awarded the Croix de Guerre for bravery under fire in Alsace. Subsequently he became a liaison officer with the Americans at Strasbourg, and at the end of the war went to Berlin, where he served in the Economic Section of the French military mission.

After leaving the army in 1947, Rainier took a villa at Beaulieu-sur-mer, where he carried on his affair with Gisèle Pascal. His intellectual interests were confined to zoology; he loved animals, and would later have a small private zoo in Monaco. As to his other pursuits, he enjoyed a whiff of danger, whether skiing, motor racing, piloting speedboats or spear-fishing in the Red Sea.

When he succeeded his grandfather as Prince of Monaco in May 1949, his first act was to set aside the will by which Prince Louis left half his fortune to Princess Ghislaine, his consort since 1946. He also invited his father back to Monaco, and set him up in the Hôtel de Paris.

Meanwhile, Gisèle Pascal was relegated to the role of weekend companion in a villa on Cap Ferrat. But she missed the theatrical world, and in 1953 married the actor Raymond Pellegrin. Thenceforward Rainier was obliged to look elsewhere in order to secure the succession.

The era of Grace Kelly ended with brutal suddenness on September 13 1982 when her car burst through the barriers on the Corniche road near La Turbie - in the same hills where she had once driven with Cary Grant in To Catch a Thief. Her funeral, unlike her wedding, was attended by an impressive number of dignitaries, including the King and Queen of Belgium, the Queen of Spain, the Princess of Wales and Nancy Reagan.

Rainier was devastated by his loss, and for some time afterwards seemed unable to maintain his normal control. In New York in 1983 he earned the sobriquet "Rocky" Rainier on account of his penchant for throwing punches at photographers who snapped him with various escorts.

There were rumours that he would marry Princess Ira von Furstenberg, reputedly described by Princess Margaret as too big a woman for so small a principality. Rainier himself might have reflected that his children provided more than enough family life.

Princess Caroline, his eldest child, reacted against her parents to the extent of marrying the French playboy Philippe Junot in 1978; they divorced in 1980, and the Catholic Church obliged with an annulment. Her second marriage, to Stefano Casiraghi in 1983, ended when he was killed in a speedboat accident in 1990. She married Prince Ernst August of Hanover in 1999.

Princess Stephanie, Rainier's youngest child, capped various amatory adventures by marrying Daniel Ducruet, her former bodyguard, in 1995; their union ended four years later when he was filmed making love to a stripper who had won the "Miss Bare Breasts Belgium" title; Princess Stephanie later took up with an elephant trainer, a butler and a gardener before deciding to marry a trapeze artist.

During the 1980s there was speculation that Rainier would abdicate in favour of his son, Prince Albert, born in 1958. But Albert's reluctance to marry, and an apparent lack of iron in his character, did nothing to forward his early succession. He now succeeds his father; in 2002 the constitution was changed so that the title can pass through the female line if he remains childless.

Even though the remarkable economic growth which Rainier had brought to Monaco tailed off in the later 1990s, he had certainly been one of the more successful princes of Monaco.