LONDON — Britain’s three main political parties set aside routine disagreements Thursday to issue an unusual collective warning to Scots that if they vote for independence, Scotland will lose the pound sterling as its currency.

Less than a week after Prime Minister David Cameron made an emotional appeal to Scots to reject independence, the cross-party initiative sent a more steely message, asserting that no future British government could accept currency union with an independent Scottish nation.

The coordinated message, led by the Chancellor of the Exchequer, George Osborne, signaled an escalation of the battle over the future of Scotland, which is to vote Sept. 18 on whether to end its union with the rest of Britain. Only those living in Scotland and aged 16 or older can vote.

A thumbs up would end Scotland’s 307-year-old marriage to England and Wales and cause the biggest political shake-up in the British Isles since Ireland split from the British crown nearly a century ago. Scottish nationalists have insisted that their country, if independent, would retain the pound and other symbols of British identity, such as the monarchy. But Osborne ruled out such an option.

Save 75% on a Digital Subscription Today

Osborne, a Conservative, was immediately backed by his counterparts in the opposition Labour Party and in the Liberal Democrat party, the junior partner in Britain’s coalition government. The unusual show of unity was intended to put Scots on notice that, no matter which of the parties holds power in London after a general election next year, none will support the use of the pound by Scotland if it secedes.

Although polls show a majority of Scots want to remain with the rest of Britain, that margin has been narrowing in recent weeks, encouraging supporters of independence led by the country’s top political leader, First Minister Alex Salmond.

Last Friday Cameron told Scots: “We want you to stay” in a speech interpreted by the British news media as an attempt to “love-bomb” the Scots into rejecting independence.

Fear of the economic unknown could sway voters to vote “no,” making the pound an important battleground in the campaign.

“There’s no legal reason why the rest of the United Kingdom would need to share its currency with Scotland,” Osborne said Thursday in a speech in Edinburgh, while citing the example of the eurozone as a lesson in the difficulties of sharing a currency without political union. “If Scotland walks away from the U.K., it walks away from the U.K. pound,” he added.

By contrast, Salmond has assured Scots they could negotiate to keep the currency if they were to vote for independence.