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By Alastair Jamieson

LONDON — Britons went to the polls on Thursday in the U.K.'s tightest general election in a generation, with observers predicting no single party would win enough support to govern Europe’s second-largest economy.

Historic buildings including windmills and pubs were among the more unusual polling locations for Britain’s 45 million registered voters to cast their ballots ahead of the 10 p.m. (5 p.m. ET) deadline.

Prime Minister David Cameron was among the political leaders to vote early in the day, visiting his local polling station in rural Oxfordshire with his wife, Samantha.

His main rival, Ed Miliband, leader of the leftist Labour Party, said the race was "the closest we have ever seen."

"It is going to go down to the wire," he told a rally on northern England late Wednesday.

Most results were not expected until early Friday, and it could be days before a new government emerges.

Cameron's Conservative Party entered a coalition deal with the center-left Liberal Democrats in 2010. The current prime minister is hoping for an outright victory this time around — but if his party falls short in this poll, Britain would be headed for only its second coalition government since World War II.

As many as one in four voters could still change their mind at the polling both, according to one survey.

Despite months of campaigning, neither Cameron nor Miliband are predicted to win the necessary number of House of Commons seats to govern alone.

Instead, voters have drifted towards smaller parties including the right-wing U.K. Independence Party (UKIP), which wants Britain to quit the European Union, and Scottish nationalists who last year narrowly lost a referendum on whether Scotland should become an independent country, ending the U.K. in its current form.

Related: UK General Election Q&A: What You Need to Know

Two British newspapers, the Daily Mail and the Guardian, on Wednesday published voting maps of Britain showing how tactical local choices could change the national outcome.

While domestic issues such as the huge deficit and the management of the state-run healthcare system have dominated the campaign, the poll still has consequences for Britain’s overseas allies — including the United States.

Cameron has promised to hold a referendum on whether to stay in or quit the European Union if he returns to power — a pledge that has unsettled markets even though a British exit from the trading bloc appears unlikely.

In addition, none of the parties has committed to extra spending on defense, cementing Britain’s recent military cutbacks and limiting its contribution to U.S.-led NATO operations.

There could even be disagreement about what to do if there is no clear winner when results trickle in early Friday.

Convention dictates that the party closest to the magic majority figure of 326 out of 650 members of the House of Commons has the first chance of trying to form an administration.

However, if the two main parties are separated by only a handful of seats, both may claim the right to govern with the backing of other groupings, setting up potentially protracted standoff.

Reuters contributed to this report.