Factbox: UK rules for inconclusive election

(Reuters) - Britain votes Thursday in what polls predict will be the closest election for almost 20 years, with the prospect of neither of the two largest parties winning an outright majority in the 650-seat parliament.

Britain has not had an inconclusive election, producing a "hung parliament," since 1974 and officials have dusted off the guidelines on how to form a government.

The new parliament sits for the first time on May 18 to elect a speaker and swear in members. The first test of any new government would be when it presents its legislative program in the Queen's speech on May 25.

Excerpts from the official guidelines on a "hung parliament" follow:

-- Where an election does not result in a clear majority for a single party, the incumbent government remains in office unless and until the prime minister tenders his and the government's resignation to the monarch.

An incumbent government is entitled to await the meeting of the new parliament to see if it can command the confidence of the House of Commons or to resign if it becomes clear that it is unlikely to command that confidence.

If a government is defeated on a motion of confidence in the House of Commons, a prime minister is expected to tender the government's resignation immediately.

A motion of confidence may be tabled by the opposition, or may be a measure which the government has previously said will be a test of the House's confidence in it. Votes on the Queen's Speech have traditionally been regarded as motions of confidence.

-- If the prime minister and government resign at any stage ... the person who appears to be most likely to command the confidence of the House of Commons will be asked by the monarch to form a government. Where a range of different administrations could potentially be formed, the expectation is that discussions will take place between political parties on who should form the next Government. The monarch would not expect to become involved in such discussions, although the political parties and the cabinet secretary would have a role in ensuring that the Palace is informed of progress.

-- A prime minister may request that the monarch dissolves Parliament and hold a further election. The monarch is not bound to accept such a request, especially when such a request is made soon after a previous dissolution.

In those circumstances, the monarch would normally wish the parties to ascertain that there was no potential government that could command the confidence of the House of Commons before granting a dissolution.

-- It is open to the prime minister to ask the cabinet secretary to support the government's discussions with opposition or minority parties on the formation of a government. If opposition parties request similar support for their discussions with each other or with the government, this can be provided by the Cabinet Office with the authorization of the prime minister.

-- As long as there is significant doubt whether the government has the confidence of the House of Commons, it would be prudent for it to observe discretion about taking significant decisions, as per the pre-election period. The normal and essential business of government at all levels, however, will need to be carried out.