A warning by Britain's three main political parties that they would rule out a currency union with an independent Scotland has "backfired" as a new poll shows that only slightly more than a third of voters believe Westminster, according to a leading psephologist.

As David Cameron tries to win over wavering voters by holding the first meeting of his cabinet in Scotland next week, John Curtice said that the gamble on a currency union had failed to boost the pro-UK camp amid signs of an increase in support for independence.

The professor of politics at Strathclyde University issued his warning as Alex Salmond said that George Osborne's decision to rule out a currency union in a speech in Edinburgh last Thursday had "backfired in spectacular fashion".

Salmond told MSPs: "Most people in Scotland would feel that George Osborne insulted the intelligence of the Scottish people … The indications we have so far is that the joint enterprise between George Osborne and Ed Balls has backfired on the two unionist parties in spectacular fashion."

The first minister turned on Labour, the Tories and the Liberal Democrats after a poll in the Scottish Daily Mail showed an increase in support for the pro-independence side in the week since the chancellor's speech. The Survation poll found that 38% support independence (up six points on a poll two weeks ago) compared with 47% who support the UK, down five points. Survation cautions that a direct comparison cannot be made between its two polls because of a change in its methodology.

The poll also found that only 37% of voters believe that the Westminster parties mean what they say, suggesting that Salmond's attack on Osborne for "bluff" is striking a chord. Curtice pointed out that only 48% of voters want to form a currency union anyway.

Curtice said that the strategy of the pro-UK Better Together campaign – to create a game-changing moment by ensuring that Britain's three main political parties ruled out a currency union on the same day – has failed. Curtice wrote on the BBC website: "The poll's headline findings suggest that, if anything, the 'no' side's stratagem has not only failed to deliver any immediate boost to the unionist cause, but has actually backfired."

Salmond seized on the Daily Mail as he joked that he was breaking the habit of a lifetime to quote from the pro-Union right wing paper. The first minister dismissed attacks from the Scottish Labour leader Johann Lamont by saying that Labour had inflicted severe damage on itself by aligning itself with Osborne.

He said: "The Labour party have done themselves huge damage by associating with the Conservatives and in particular George Osborne. The reaction of the Scottish people to being told, instructed from on high that our currency – the currency that we jointly built up – actually doesn't belong to us, it belongs to George, is entirely understandable and will be deeply uncomfortable for the Labour party in Scotland."

Salmond was scathing about José Manuel Barroso, president of the European commission, who said it would "difficult if not impossible" for Scotland to join the EU because Spain might block its accession bid. Barroso pointed out that Spain, which fears secession by Catalonia, has blocked Kosovo's membership bid because it broke away from Serbia. Salmond highlighted remarks by Jim Currie, a former director general for environment directorate at the European commission, who described Barroso's remarks as "extremely unwise" and "inaccurate".