The fate of the United Kingdom was at stake Thursday as Scotland began voting in a referendum on becoming an independent state, deciding whether to unravel a marriage that helped build an empire but has increasingly been felt by many Scots as stifling and one-sided.

The question on the ballot paper is simplicity itself: ‘‘Should Scotland be an independent country?’’ Yet it has divided Scots during months of campaigning — and in 15 hours on Thursday the future of the 307-year old union with England hangs in the balance.

More than 2,600 polling places opened Thursday at 7 a.m. (2 a.m. EDT) and will close at 10 p.m. (5 p.m. EDT). Turnout is expected to be high, with more than 4.2 million people registered to vote — 97 percent of those eligible.

Polls suggest the result is too close to call, with the pro-independence Yes side gaining momentum in the final weeks of the campaign.

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