Foreign diplomats believe Scotland is likely to vote for independence after a series of opinion polls have shown an increase in support for a yes vote in September's referendum.

Sources in the diplomatic corps in Edinburgh, which is home to nearly 50 consulates and diplomatic missions, have told the Guardian they think the tide of opinion has shifted significantly in recent months, after a noticeable swing against David Cameron's government and the no campaign.

One senior diplomat, who asked not to be named, said he had believed last year that a yes vote was unlikely, but had since changed his mind. In his view "it is now likely, but not certain" that Scotland would vote yes in September, he said.

Their intervention will boost the yes campaign just as the Scottish National party gathers in Aberdeen for its spring conference this weekend, its last before September's referendum.

Alex Salmond, the first minister and SNP leader, is expected to point to the latest polls as proof the SNP is poised to win its historic goal of independence, in its 80th year since its foundation in 1934.

But the Treasury sought to dampen that mood by publishing fresh data from the International Monetary Fund which it said showed that Scotland would have one of the highest spending deficits of all advanced economies if it became independent in 2016, at £9.5bn.

The Treasury said that deficit, equivalent to 5.5% of Scotland's GDP or £1,760 per head – £1,000 per head more than the UK's projected annual deficit that year – would be put immense pressure on Scottish public spending.

Two opinion polls on Thursday confirmed the gap between a yes and no vote has narrowed. A Survation survey for the Daily Record and Dundee University put yes at 44% and no at 56%, a gap of 12 points, excluding don't knows. A second, by Panelbase for the pro-independence Yes Scotland campaign, confirmed Panelbase's other recent polls by putting the yes vote at 47% and the no vote at 53% after excluding undecideds – one of the narrowest margins in recent months.

While backing for independence in the Survation poll was slightly down on its poll last month, Yes Scotland said the average of all recent polls put a yes vote at 46% and no at 54% – a difference of eight points compared with a 38% to 62% gap last November.

A second foreign government source said most diplomats did not believe the UK government's stance on key questions such as George Osborne's claims the Treasury would veto a currency union, that Scotland's economy would suffer badly after independence and that Scotland would be barred from immediate EU membership.

Diplomats instead believed the UK government's hostility on the currency, immigration and Europe was creating a backlash among Scottish voters. Most diplomats, who have yet to see a significant push for a no vote by the anti-independence campaigns, believed the chancellor would agree a currency pact after a yes vote.

"The UK government's policies are pushing Scotland away," said one European consul. "My hunch is that unless the UK government radically rethinks, it's on a hiding to nothing. It is losing the argument."

Their scepticism about Osborne's stance on sterling was underlined by the international credit ratings agency Fitch in a report on Thursday.

Fitch said it doubted Scotland would vote for independence but a yes vote had to be carefully considered as "a potentially relevant event for the remaining UK". It would have a "significant" effects on the UK and the Westminster government would want to make sure independence caused as little disruption as possible to the economy.

It predicted the UK would compromise with Scotland on key economic issues, since Scottish independence would see the UK's debt increase by nearly 10%. It would also be "mildly negative" for the UK's trade balances because it would lose nearly all its North Sea exports, although that was unlikely to affect the UK's credit rating of AA+.

The pro-UK Better Together campaign and Scottish Labour argue privately that since their main campaigns against independence have not yet started, the latest opinion polls only reflect a temporary public mood.

Yes Scotland has launched its nationwide billboard advertising campaign in favour of independence: Better Together's national campaign will not launch till next month, closer to the start of the formal referendum campaign on 30 May.

The chief secretary to the Treasury, Danny Alexander, said the IMF figures showed independence "would be a great risk to the Scottish economy, and would mean higher tax bills and cuts to public services to balance the books". He added: "Being part of the larger UK economy provides Scotland with jobs, stability and security. Independence would mean higher taxes and lower spending on public services."