EDINBURGH, Scotland — Scots held the fate of the United Kingdom in their hands Thursday as they voted in a referendum on becoming an independent state, deciding whether to unravel a marriage with England that built 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 polls suggest the result is too close to call.

A final Ipsos MORI poll released Thursday put support for the No side at 53 percent and Yes at 47 percent. The phone survey of 991 people has a margin of error of plus or minus three percentage points.

Until recently, polls suggested as many as one in five voters was undecided, but that number has shrunk dramatically. In the latest poll, only 4 percent remained uncertain how they would vote.

Voters lined up outside some polling stations even before they opened at 7 a.m., and on the fog-shrouded streets ofScotland’s capital, Edinburgh, there was a quiet thrum of excitement at history unfolding — an electric mood tinged with nervousness.

The future of the 307-year-old union with England will be decided in 15 hours on Thursday; polls close at 10 p.m. (2100 GMT, 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. Citizens and residents as young as 16 can vote.

A Yes vote would trigger 18 months of negotiations between Scottish leaders and London-based politicians on how the two countries would separate their institutions ahead of Scotland’s planned Independence Day of March 24, 2016.