By Jason Douglas And Josie Cox 

LONDON--A new poll Tuesday showed a surge in support for Scottish independence, dramatically raising the stakes for Britain's leaders just weeks before Scots vote on whether to leave a 300-year-old union with the rest of the U.K.

Pollster YouGov said that in its latest poll of more than 1,000 Scots, 42% said they intend to vote for independence in a referendum Sept. 18, while 48% said they plan to vote to keep Scotland part of Britain. The remainder were either undecided or said they won't take part.

YouGov's latest poll was carried out between Aug. 28 and Sept. 1.

A poll earlier in August put the 'no' lead at 12 points.

The findings--from a polling company whose past results have shown a convincing lead for the pro-U.K. 'no' camp--are likely to ring alarm bells in London, where all three main political parties oppose Scottish independence.

The results will be greeted with jubilation in the pro-independence 'yes' camp, spearheaded by the Scottish National Party, which holds power in Scotland's semiautonomous parliament in Edinburgh.

"A close finish now looks likely, and a 'yes' victory is now a real possibility," said Peter Kellner, YouGov President, in a blog on his firm's website.

The pickup in support for the 'yes' campaign follows weeks of intense campaigning by both sides, including a bad-tempered televised debate between Alex Salmond, the chief minister in Scotland's government and leader of the SNP, and Alistair Darling, a Scot and former U.K. finance minister who leads the pro-U.K. Better Together campaign. A snap poll published after the debate showed a majority of viewers thought Mr. Salmond the winner.

It also follows an intervention by business leaders, more than 100 of whom signed a letter saying independence would be bad for Scotland' economy.

Yet YouGov's poll showed Scots' nervousness about the economic consequences of quitting the U.K. has diminished, despite warnings from London that an independent Scotland would be considerably worse off than if the nation stayed in the U.K. YouGov reported that 35% of Scots thought Scotland would be better off outside the U.K, up from 32% in August. Mr. Salmond has long argued Scotland's oil wealth would ensure the country would prosper if Scots choose to secede.

In financial markets, however, the increasing prospect of Scotland leaving the U.K. is unsettling investors.

BlackRock Inc., the world's largest asset management firm with about $4.3 trillion in assets under management, last week said it is now using derivatives to bet against sterling heading into the vote.

The chief investment officer of Kleinwort Benson, which has GBP5.7 billion under management, meanwhile, has warned that a winning independence vote could easily see the pound collapse.

In early trade Tuesday, the pound dipped 0.5% against the U.S. dollar to $1.6539.

In bond markets, the U.K's December Gilt slipped a third of a point lower to 113.06, according to Tradeweb. 10-year U.K. government bonds currently yield 2.419%.

Write to Josie Cox at josie.cox@wsj.com

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