For the past half-century, Richard the Lionheart - that buff, bronzed warrior who hardly saw his wife and had no children - has been something of a gay icon. As a presence on the silver screen (most famously in the shape of the young Anthony Hopkins in The Lion in Winter) his homosexuality has rarely been in doubt.

English history isn't short of gay or bisexual monarchs - Edward II, James I, possibly William II - but the historical evidence for counting Richard I among their number rests on one contemporary document concerning his relationship with King Philip II of France. In 1187, a chronicler reports, the two men were so close that "at night the bed did not separate them".

Now, however, as the BBC prepares to air a new Lionheart docu-drama, the king's biographer, Professor John Gillingham, has pointed out that Richard's ostentatious bed-sharing with the French king was the product of a political alliance rather than a lovers' tryst.

Gillingham's suggestion that this was "an accepted political act, nothing sexual about it" might strain modern credulity - but we should remember that diplomacy has always been intensely personal, if not downright physical. Only this week, Jonathan Powell's account of the Northern Irish peace process has highlighted the significance of Tony Blair's decision to shake hands with Gerry Adams - the press of prime ministerial flesh on republican palm a powerful gesture of political intent.

In centuries past, a wider range of body parts might come into diplomatic play. Medieval rulers, for example, routinely greeted one another with a kiss (the biblically sanctioned "kiss of peace"). Richard's decision to share a mattress with Philip was the ultimate public demonstration of trust in an age when PR had to rely on word-of-mouth rather than the lenses of the international media.

And it worked, in the context of a monarchy where privacy was relative and political life didn't stop at the bedroom door. The king held court in his bedchamber, and his favourite servants slept at the foot of his bed. World leaders don't, any more, feel the need to ratify a treaty by getting into bed with each other - though, interestingly, that's still the language we use when we talk of sealing a deal. Perhaps we should just be grateful that, these days, the "special relationship" between the UK and the US doesn't involve seeing Bush and Brown in their underpants.

· Helen Castor is a medieval historian and author of Blood & Roses: the Paston Family and the Wars of the Roses.