The International Monetary Fund has warned that a yes vote in next week's Scottish independence referendum could result in financial market turmoil.

A vote for independence would create "uncertainty" while a number of "complicated issues" were being thrashed out, in particular over which currency an independent Scotland would use, the Washington-based organisation said.

The long-term impact on the economy would be determined by the outcome of the detailed negotiations carried out in the aftermath of the referendum, which is less than a week away.

The IMF deputy spokesman, William Murray, said at a press briefing on Thursday evening: "A yes vote would raise a number of important and complicated issues that would have to be negotiated. The main immediate effect is likely to be uncertainty over the transition to potentially new and different monetary, financial, and fiscal frameworks in Scotland.

"While this uncertainty could lead to negative market reactions in the short-term, longer-term effects would depend on the decisions being made during the transition. And I would not want to speculate on this."

The warning came as a new YouGov poll showed support for separation weakening by three percentage points.

The latest poll for the Times and the Sun found that support for remaining in the UK has risen to 52%, leaving support for a yes vote four points behind at 48%, excluding don't knows.

A YouGov poll last week showed the yes vote leading for the first time, taking a two-point lead over no by 51% to 49%, sending shockwaves through the no campaign and causing delight among yes campaigners. That poll led to a fall in the value of the pound, and to more than £2bn being temporarily wiped off the value of leading Scottish companies. It also forced David Cameron, Ed Miliband and Nick Clegg to abandon prime minister's questions and head to Scotland for a day of campaigning to shore up the no vote.

The latest YouGov panel has shown a shift against independence in several areas. It found that the yes vote was ahead in only one age group, among 25- to 39-year-olds, while in all others the no campaign was ahead. Its previous poll for the Sunday Times found the no vote was ahead just among pensioners.

It also found that first minister Alex Salmond's trust ratings were down four points since 5 September, from 42% to 38%, while the ratings for the new Labour no-vote champion, former prime minister Gordon Brown, were up by three points to 35%.

The gap among those who felt Scotland would be poorer had grown after 45% said it would be worse off after independence, compared with 37% six days ago. Only 21% felt it would be richer after a yes vote, against 23% last week.

On Wednesday, the pollster Survation gave the no campaign a six-point lead, at 53% to 47% – the same level both campaigns had been at two months ago. With further polls due over coming days, that flattening trend for yes has now been confirmed by YouGov.

"This is the first time no has gained ground since early August," the polling organisation said. "Three previous polls over the past month had recorded successive four-point increases in backing for independence. In early August, yes support stood at 39%; by last weekend it had climbed to 51%.

"Although no is back in front, the yes campaign has held on to most of its gains since early August. These gains have come partly from people switching from no to yes and partly from don't knows."

Blair Jenkins, chief executive of the pro-independence campaign Yes Scotland, insisted he was not disappointed and said this poll confirmed the two camps were neck and neck. "We are in touching distance of success next Thursday, and there is everything to play for," he said. "It will spur on everybody who wants, and is working hard for, a yes to redouble their efforts.

"As we say in response to all the polls, we are working flat out to ensure that we achieve a yes vote, because it's the biggest opportunity the people of Scotland will ever have to build a fairer society and more prosperous economy."

Blair McDougall, the campaign director for Better Together, said: "Every voter has the power to tip the referendum either way. This decision is far too important to be a protest vote. That is why thousands of Scots will be pounding the streets over the next week to make sure that we win the referendum.

"We have seen in the past few days, in all too stark reality, exactly what would happen if we leave the UK. Jobs would go, companies would move south, and prices would rise in our shops. When we have the chance to vote for more powers within the security of the UK, why would you take that risk?"