When Britain votes on its national election Thursday, the Labor and Conservative parties each will be trying to win a majority of the 635 seats in the House of Commons, with the winning party's leader becoming prime minster.

Labor won a bare majority of 318 seats in the last national election in October 1974, but losses in by-elections to fill seats that became vacant during the past five years reduced Labor's strength to just 309 seats, compared to 282 held by the Conservatives, 14 by the Liberals, 12 by various sectarian parties in Northern Ireland, 11 by the Scottish Nationalists, and 3 by the Welsh nationalist Plaid Cymru party.

Labor's lack of an overall majority eventually made it possible for a combination of the Conservatives, Liberals, Scottish Nationalists and most of the Northern Ireland members to bring down the Labor government on a no-confidence vote at the end of March and force Thursday's election.

If neither Labor nor the Conservative Party wins an overall majority, either the party with the most seats or the party that can attract enough support from the minor parties to create a majority coalition will try to form a government.

The election is expected to produce unusually strong regional patterns, with Labor sweeping the overwhelming majority of seats in Scotland, Northern England and Wales, and the Conservatives winning the lion's share of seats in southern and central England.

The closely contested industrial midlands of north central England, around Manchester, Birmingham and Leicester, could be decisive.

Because of the strong regional variations, the traditional last-minute changeability of swing voters, and the fact that the government is selected by election individual members of the House of Commons rather than voting directly for the prime minister, public opinion polls have been notoriously poor predictors of previous elections here. The opinion polls predicted a Labor Landslide in 1970, when the Conservatives won, and they forecast that the Conservatives would stay in power in 1974, when they were ousted by Labor.

If the election is close, it could take two days to find out who won because ballot counting will not begin in more than 100 of the 635 parliamentary constituencies, most of them rural, until Friday. Counting is expected to be particularly slow in areas where local council elections for the first time are being held on the same day as national elections