Michael Martin's successor will be selected through a secret ballot for the first time.

The new rules were introduced in 2001 after the election which brought Mr Martin to the chair turned into a shambolic seven-hour marathon.

Candidates will need the backing of between 12 and 15 other MPs, - at least three of them from other parties.  Each will try to win support with a speech to the Commons, before MPs fill in their ballot papers and cast their votes.

If any candidate gets more than half the votes, they will be elected Speaker. If no-one does, then the hopeful with the fewest votes and any with less than 5% of the total will be eliminated and the remainder voted on afresh until someone eventually enjoys an overall majority.

The last two Speakers, Betty Boothroyd and Mr Martin, have both been from Labour, ending a tradition - admittedly only a decades rather than centuries-old one - of alternation between the main parties.

The Tories would be keen to have one of their own fill the chair this time. Although the Speaker is formally outside party politics and represents no party in the constituency.

But if the election is held before the next General Election, which opinion polls firmly suggest the Tories will win, Labour will retain a Commons majority and could secure a third consecutive incumbent.

On winning an absolute majority in the Commons, the Speaker-elect is expected to show reluctance at being chosen and is customarily "dragged" by colleagues to the Chair - a nod to the dire fate that has befallen some speakers down the centuries.

The Speaker-elect must then receive the Sovereign's approval, or the approbation, before he or she may take office.

Once in office, the Speaker must be politically impartial and will resign from their political party.

The Speaker remains separate from political issues, even in retirement, but will deal with their constituents' problems like a normal MP.

Speakers also tend to be unopposed in their constituency during a General Election, with the major parties not fielding a candidate and the Speaker not campaigning on any political issues, but simply standing as "the Speaker seeking re-election".