What is prime minister's questions?

David Cameron spent more than four years asking the questions before becoming PM

Prime minister's questions or "PMQs" is a high point of the parliamentary week.

Each week on Wednesday afternoon the prime minister must come to the House of Commons to answer oral questions for half an hour.

This system was changed by Tony Blair's Labour government shortly after they came to power in May 1997. Previously PMQs took place on Tuesday and Thursday afternoons for 15 minutes.

Traditionally the leader of the opposition may ask up to six questions during the session and the leader of the Liberal Democrats two questions.

But Nick Clegg's Lib Dems are now part of a coalition government so he will no longer get the chance to ask questions at this showpiece event.

Instead he will stand in for Conservative Prime Minister David Cameron when he is not present, and will also get his own monthly deputy prime minister's questions.

It is expected that because the Lib Dems will not get to ask questions, the smaller opposition parties may get a chance to put the PM on the spot on a more regular basis.

Prime minister's questions follows a different format to those of questions to other ministers. MPs do not normally give the prime minister prior notice of the subject which they are going to raise.

This element of surprise allows opposition MPs, in particular, to try to catch the prime minister out with an awkward question. The prime minister must respond without delay, thinking on his or her feet.

Government backbenchers can normally be relied upon to ask a "helpful" question which will allow the prime minister to tell the House about successful government policies.

The relative performance of each of the main party leaders is closely watched and each is under great pressure to get the better of their opponent.

The chance to ask the prime minister a question is highly prized. The names of the MPs who will get the chance to ask the prime minister a question are drawn in a weekly lottery.