Key reasons put forward by the UK government for rejecting a currency union with an independent Scotland are unsubstantiated, an economist has claimed.

The chancellor, George Osborne, cited Treasury advice when he ruled out sharing the pound in the event of a yes vote in this year's independence referendum. But Leslie Young, economics professor at the Cheung Kong Graduate School of Business in Beijing, said the arguments could not withstand scrutiny.

The Scottish government in Edinburgh has put forward plans to retain the pound if there is a yes vote in September, arguing that this would be to the benefit of both Scotland and the rest of the UK.

But the chancellor and his Labour and Liberal Democrat counterparts, Ed Balls and Danny Alexander, have all ruled out such a deal. In the wake of a speech in Edinburgh this year, Osborne published a letter from his permanent secretary summarising Treasury advice on the currency union issue.

Young said: "There may be good reasons for the UK to reject a currency union with an independent Scotland, but none can be found in the Treasury letter. Yet, that letter is the key justification for the stance of the UK government."

The Scottish government welcomed the report while the Treasury maintained that a currency union was "not going to happen".

Young challenged each paragraph of the Treasury letter in his report, describing it as "loose analysis" with "inconsistent assumptions".

He said: "[The letter] does not even address the question that it purports to answer: whether the currency union is in the interests of the UK, if Scotland voted for independence.

"The Treasury claims are invalidated, not by errors of fact, but by errors of logic. These errors are subtle and difficult to disentangle. But only subtle logical error could have led Treasury to claim, in effect, that past risky behaviour by investment bankers in London, inadequately supervised by the Bank of England, somehow disqualifies an independent Scotland to be a currency union partner of England."

A spokesman for Scotland's first minister said the report "totally demolishes" the Treasury's argument against a shared currency. He said: "As the fiscal commission working group has pointed out, the UK government analysis to date has overstated the risks but failed to fully capture the benefits of formal monetary union."

A Treasury spokesman said: "A currency union is not going to happen … This decision is not going to change. This means less than six months from the referendum the Scottish government still has no plan for what currency they would use."

The Yes Scotland chief executive, Blair Jenkins, said: "As most Scots know, the pound belongs to Scotland every bit as much as the rest of the UK and this forensic analysis from a respected economist exposes the fact that the anti-independence parties' opposition to a currency union is purely political and designed to scare people into voting no – the rise in support for yes shows it is backfiring.

"As it is now abundantly clear, a formal currency union makes sense not only for Scotland, but for the rest of the UK as well."

A spokesman for the pro-union Better Together group said: "This is a flawed piece of work which completely fails to take into account the extensive analysis produced by the Treasury's expert economists.

"A currency union isn't going to happen. As Professor Young points out, Alex Salmond has a duty to tell us his Plan B for what would replace the pound. Would we rush to adopt the euro or would we set up a separate unproven currency? The idea that Scots can go to the polls in September blind on this most fundamental issue simply isn't credible."

Meanwhile the number of Scots ready to vote for independence has risen, according to a poll published on Sunday, adding to evidence that the referendum on 18 September could be tighter than previously expected.

The poll, conducted by ICM, showed the proportion of Scots who would vote in favour of independence rose to 39%, an increase of two percentage points from a similar survey conducted last month. The number of those voting against a split fell to 46% from 49% in February, according to the poll of 1,010 people.