Support for Scottish independence is at its highest level since last summer, according to a new poll.

Campaigners for a Yes vote in the referendum would have to secure a swing of about three points to win on September 18, the Panelbase results indicate.

The poll suggests support for Scotland to leave the UK stands at 40% - the highest since August - compared with 45% for the country to stay in the union, with 15% still undecided.

When those undecided voters are excluded, the results are 47% for Yes and 53% for No, the narrowest gap since last October.

Polling expert Professor John Curtice, of Strathclyde University, said the increase is modest but consistent with other polls.

"There can now be little doubt that the No side's lead has narrowed - and equally that last month's currency intervention has so far failed to reverse that trend," he wrote on the What Scotland Thinks website.

Chancellor George Osborne, backed by the opposition at Westminster, insisted last month that Scotland would not be able to join a formal currency union with the rest of the UK after independence. A so-called sterling zone is a key Scottish Government proposal for independence.

Scotland's Deputy First Minister, Nicola Sturgeon, said: "This is another remarkable poll which shows increasing support for a Yes vote in September - the highest level of support for an independent Scotland for over a year. Only a three-point swing is needed to put Yes ahead.

"This builds on the Survation poll last week that also showed there is significant movement towards Yes. It is increasingly clear that the more people engage with the debate the more likely they are to vote for independence, and that the momentum in the referendum campaign is with the Yes campaign."

Yes Scotland chief executive Blair Jenkins said: "The scaremongering of the No campaign is backfiring because people understand that as the 14th wealthiest country in the international league table, compared to the UK's 18th place, Scotland has got what it takes to be a successful independent country."

But a Better Together spokesman dismissed the results.

"A similar nationalist-commissioned Panelbase poll back in September had Yes in the lead so this is hardly progress for the nationalists," the spokesman said.

Panelbase, commissioned by the independence-supporting website Newsnet Scotland, surveyed 1,036 people between March 7 and 14.