John Wilkes By Arthur H Cash

"Wilkes and Liberty!" It may mean nothing now, but 250 years ago you would have been in front of St James's Palace, shouting it at the top of your voice. John Wilkes is the father of the political system we have today and a major influence on that adopted by America: he established freedom of the press as we know it, argued for yearly elections and the abolition of rotten boroughs, and was the first MP to propose universal suffrage in the Commons. In this much-needed biography, Arthur Cash tells the story of an unlikely hero of the people.

Wilkes was a gentleman, trained lawyer, and something of a scholar, which already put him halfway down the road to being a libertine - which he also was. As a young man, he joined the Knights of St Francis who met at Medmenham Abbey near Marlow to have orgies with assorted "courtesans", as Cash calls them - "always ladies".

This book stands as a warning to bullies, for it illustrates the old dictum that the fastest way to radicalise moderates is to oppress them. Wilkes didn't start out with any feeling for working people but realised his destiny when, in May 1763, he was arrested as the suspected author of a newspaper, the North Briton, which made mincemeat of the royal prerogative. Desperate to find ways of getting round the privileges Wilkes claimed as an MP, George III's ministers used a very large sledgehammer to smash a nut - arresting 49 people, confiscating the contents of Wilkes's house, and locking him for years in the Tower of London. Nor did they hesitate, again and again, to bend the letter of the law to find evidence to keep him there, including the theft of Wilkes's bawdy poem, "An Essay on Woman".

It was the kind of mistake made by tyrants everywhere. Realising he had nothing to lose, Wilkes dug in and slogged it out. Imprisonment turned him into a martyr for human rights and, in the Tower, he got himself re-elected to Parliament even after the Commons had declared him "incapacitated". He became so popular that his image appeared on cigarette papers, dishes, punchbowls, teapots, and penny-ballads - a sort of 18th-century cross between Nelson Mandela and Che Guevara.

Besides losing his freedom, he suffered exile and financial ruin for his beliefs. But he was successful in rescinding the law by which he had been arrested, preventing Parliament ever again from "incapacitating" its members, and helping secure freedom of expression for succeeding generations.

Cash tells Wilkes's story with gusto, breathing life into the most intricate details, fusing them into a fast-paced narrative. He is helped by a starry supporting cast that includes Sterne, Rousseau, Hume, Dr Johnson, Boswell and Voltaire. Even the walk-ons are colourful, not least the transvestite French diplomat, Mademoiselle Eon, who became lady-in-waiting to Marie Antoinette before returning to London as proprietor of a fencing academy. Her true sex (male) was not discovered until after her death in 1810.

Wilkes's female companions include some who sound as if they had stepped out of The Beggar's Opera: Lucy Ballards, Kitty Towlers and Fanny Perfect (not her real name, surely?), as well as the more exotic-sounding Gertrude Corradini, Mlle Chassagne, and Marianne de Charpillon. The pages of this book sizzle with intrigue and indecency. It's a bracing read, not for the faint of heart. In this respect, Cash is to be congratulated. Unlike so many academics, who pelt the idol of political correctness with rotten eggs in private while kneeling at its shrine in public, he is an excellent, plain-spoken writer unafraid of calling things by their names.

The Lord Mayor of London was "not very bright"; Sterne's daughter was "a ridiculous woman"; Mrs Otto "lost her place in history" because she declined to succumb to Wilkes's charms; music teachers are (to Cash) "notoriously poor and unstable". Wilkes himself was "free from cock to wig", called his manhood "pego", and would not "deny him his carnival". He reveals that in the 18th century English and French women never undressed when in the act - indeed, you could be arrested for doing it in the nude.

Cash's biography may not amuse you if you're a feminist, if you think smutty books should be banned, or if you think the monarch is above criticism. But if you care at all about why you have the liberties you do (imperfect though they may be), you should read the story of John Wilkes, godfather of urban radicalism.

Duncan Wu is professor of English at Oxford University