Primed and primped in back-rooms by their trainers, their eyes gleaming, the two men faced each other.

Just a few feet separated the glowering heavyweights. On packed green benches, men and women cheered or booed as their champion stood for combat.

The cameras poised like vultures to capture every feint, dive, jab and thrust for the eager public.

First to lay a glove was Ed Miliband, head of Her Majesty's Loyal Opposition.

Did the House know, he asked in a sweetly reasonable tone, that the Prime Minister's plans to reform the welfare system would deprive 7000 British cancer victims of £94 ($188) a week in sickness benefit?

Labour heads nodded. With an eye on the welfare reform bill due to be debated that same night, Miliband had asked a detailed question on how benefits would be administered for patients who took a longer time than expected to recover from cancer.

By all the laws of politics, David Cameron should have moved smartly to one side. Never talk in detail on a subject that you know little about - especially when the key words are "cancer", "victim" and "deprive," the political equivalent of a hand grenade with the pin removed.

But the normally shrewd Old Etonian walked right into the trap. He attacked Miliband for refusing welfare reform, blasted him as the "weak leader of a divided party" then, challenged again and again, did something astonishing. In desperation, he read from a binder where he had his briefing notes.

"Let me explain the definition of who is terminally ill," he said. "Let me explain! These are horrid things to have to discuss!"

All of a sudden, a look came over Cameron as he realised he had been painting himself, to a nationwide audience, as a bean-counter with a heart as cold as Antarctica.

On the Opposition benches, Labour MPs cackled and hooted like harpies at an all-you-can-eat carrion party. In pain, the Tories screeched back, denouncing Labour as the party to blame for the welfare mess in the first place.

The Speaker of the House, John Bercow, called for calm.

"It's a disgrace that members on both sides of the House are shouting their heads off when matters of the most serious concern are being debated," Bercow said tetchily. "The public despise this sort of behaviour. Let's have a bit of order."

But Mr Speaker was wrong. The British love a political punch-up, and the surest place to find it is the weekly House of Commons slot when the Premier has to take on allcomers.

For 30 minutes, he or she has to face half a dozen questions. Some may be planted by party toadies and others straightforward and without political bias.

But there is always a duel with the head of the Opposition, an unscripted bout where the outcome is determined by who has the clearest head, the most silvered tongue and the best ability to marshal facts - or at least distort them most plausibly.

Prime Minister's Questions - PMQs to Westminster groupies - lies at the heart of Britain's tradition of adversarial politics.

"PMQs is more blood sport than political debate, boxing with words, not gloves - and the goal is a knock-out punch or at least to draw blood," said Anne Treneman, parliamentary sketch-writer for the Times.

"It is the big set-piece of the political week and, in the time just before, everyone is flooding into the chamber and the atmosphere becomes more and more febrile. David Cameron used to deride it as a Punch and Judy show, but these days he punches with the best of them.

"It can be immensely exciting to watch: the roar of the crowd, two men going head to head, backed by tribal whoops and jeers. It also can be quite immature, not to say bullying. But, in the UK, it is a must for the leaders to learn to master it. Ed Miliband had a terrible PMQs two weeks ago - we have seen corpses with more life - but he was transformed this week."

British prime ministers have been answering questions in Parliament since the early 18th century, but it was only in the 1880s that the practice became codified and from the 1960s that it occupied regular slots.

Then two PMQs of 15 minutes apiece, on Tuesdays and Thursdays, became a single 30-minute bash on Wednesdays.

Other countries in the Commonwealth and continental Europe have adopted PMQs.

In Germany's Bundestag, they can be noisy affairs as parties clamour for the spotlight. But the chamber is light and airy and seating is in a hemicycle, so it lacks the Gothic bear-pit feel of the Commons.

In France, the top politician is the President, who never has to face questions anyway, except from a usually respectful media. Questions in the National Assembly are generally put to the Government, rather than the prime minister in person, are screened in advance by a joint committee and are limited to five minutes for each question.

As a result, posturing in Paris is done outside the chamber.

On the day that Miliband took on Cameron, leading French Socialist Segolene Royal mounted an attack on MPs who accused her of being a media slut with no spontaneity. She did so - you guessed it - at a press conference.