Britain's Prime Minister David Cameron and the Scottish leader Alex Salmond have signed an agreement to hold a referendum on Scottish independence.
The referendum could lead to the United Kingdom breaking up after 300 years, leaving England, Wales and Northern Ireland in the union.
Mr Cameron met First Minister Alex Salmond in Edinburgh on Monday following months of negotiations about the ballot, expected to be held in 2014.
The vote will be limited to a single Yes-No option on Scotland leaving the UK.
Proposals for a second question on further devolution, short of independence, were firmly opposed by the British government, PA reports.
The referendum will be open to 16- and 17-year-olds.
The UK government, which has responsibility over constitutional issues, will grant limited powers to the Scottish Parliament to hold a referendum, under a mechanism called Section 30.
Mr Cameron, the leader of the Conservative Party, has pledged that keeping the United Kingdom together is his No 1 priority, but said he believed in showing respect to people in Scotland.
"The people of Scotland voted for a party that wanted to have a referendum on independence. I've made sure, showing them respect, that we can have that referendum in a way that is decisive, that is legal, that is fair but crucially is one single simple question, whether Scotland wants to stay in the United Kingdom or go."
Mr Salmond secured a mandate to hold the referendum by winning an unprecedented majority with his Scottish National Party (SNP) at Holyrood last year.
He said the agreement paves the way for the most important decision Scotland has made in several hundred years.
"It is, in that sense, an historic day for Scotland and a major step forward in Scotland's home rule journey."