LONDON (AFP) - Support for Scottish independence is falling ahead of a referendum in September, with voters increasingly fearing economic fallout from a split with Britain, a poll showed Tuesday.

A YouGov poll for The Times newspaper said support for a "Yes" vote has fallen to 35 per cent, while 54 per cent would vote against independence, and 12 per cent were undecided or would not vote.

With undecided and non-voters stripped out of the survey, which was carried out at the end of June, the result of 39 per cent "Yes" is down three percent from the comparable figure in March.

A poll of polls shows that Scottish First Minister Alex Salmond's pro-independence campaign has been narrowly behind support to keep the 307-year-old union with England.

Support for independence initially rose after Mr Salmond announced his referendum plan in 2010, but has plateaued in recent months and now seems to be falling.

Speaking to AFP on Saturday, Mr Salmond was bullish.

"We're closing all the time... we've got a bit to go but we're travelling in the right direction," he said.

The economy has been a major battleground, with independence activists saying a split would benefit Scotland but unionists warning of harm to both countries.

The latest YouGov poll, which interviewed 1,206 Scottish adults between June 25 and 29, showed only 27 per cent thought Scotland would be better off as an independent country, down three points since March, while 49 per cent believed it would be worse off, up four points.

The British government has warned that Scotland would be forced to ditch the pound sterling in case of independence, and that an independent Scotland may be forced to reapply for European Union and NATO membership.

Mr Salmond says North Sea oil revenues would boost Scotland's economy.