Explorers battle through the polar ice

London, England (CNN) -- British voters go to the polls Thursday to determine the fate of Gordon Brown's Labour government, which has run the United Kingdom for the past 13 years.

The casting of ballots across England, Scotland, Wales and Northern Ireland caps a hectic month-long election campaign marked by first-ever televised debates between the leaders of the three main parties: Brown, David Cameron of the Conservative Party, and Nick Clegg of the Liberal Democrats.

Brown has stood at the center of British government since Labour ended 18 years of Conservative rule in 1997 -- as the powerful chancellor, or finance minister, under Tony Blair for a decade, and then as prime minister since 2007.

He's also the man who made the only gaffe that got any attention during the campaign. Not realizing he was still wearing a live microphone in his car as he was driven away from meeting a retired widow in northern England, he said the conversation was a "disaster."

"Should never have put me with that woman -- whose idea was that?" he said to an unidentified aide. "She was just a sort of bigoted woman."

Brown apologized repeatedly for the remark made eight days before the election, first on a live radio show shortly after he said it, and later at the home of the woman, Gillian Duffy.

British law prevents the media from reporting how the parties have fared in opinion polls during the campaign or from detailing the parties' positions on the issues. But online media are not required to remove such reporting from their archives.

The law restricts reporting on polling day to uncontroversial details such as where and when party leaders voted, and what the weather was when they did.

Polling stations are open from 7 a.m. until 10 p.m. local time (2 a.m. to 5 p.m. ET.) Many smaller parties also are competing, including nationalist parties such as the Scottish National Party, Plaid Cymru in Wales, and unionist and republican parties in Northern Ireland.

The far-right British National Party is hoping to win its first-ever seats in the House of Commons, after having won races for European Parliament seats last year. The United Kingdom Independence Party is also fielding candidates, as is the venerable if satirical Monster Raving Loony Party, whose candidates have been known to say, "Vote for insanity! You know it makes sense!"

Voters in 650 constituencies across the country choose lawmakers to represent them in the House of Commons. The candidate who gets the most votes in a constituency wins. It's not necessary to win an absolute majority of votes in a constituency to win the seat.

The leader of the party with the most seats in the Commons traditionally gets the first chance to form a government.

Due to the structure of the British voting system, one party normally wins a majority of seats, even though there are three national parties.

The last time no one party captured more than half of the seats in the Commons was in 1974. That government proved unstable, and voters were back at the polls within months.