Britain's main political parties are drawing up plans to deliver a powerful message to the Scottish people that the remainder of the UK will refuse to form a currency union with Scotland if voters endorse independence in the referendum in September.

Amid strong polling evidence that financial matters have become the decisive factor in voters' minds, George Osborne, Danny Alexander and Ed Balls are each planning to deliver an unequivocal warning that an independent Scotland could not keep the pound.

David Cameron signalled at his Downing Street press conference on Tuesday that Osborne would be first out of the blocks later this week. The chancellor will reject a currency union as he develops his warning last week that first minister Alex Salmond's proposal had been "demolished" by the Bank of England governor Mark Carney in a speech in Edinburgh last month.

The prime minister said: "The chancellor will be discussing this later in the week. I think it would be very difficult to justify a currency union post-independence. But I think it is very important that we set out these arguments in a very clear and rational way."

The Guardian understands that Danny Alexander, the Liberal Democrat chief secretary to the Treasury, and Ed Balls, the shadow chancellor, will deliver similar warnings. But the three finance spokesman for Westminster's main political parties have decided that they should speak out on their own terms and in their own time because of the acute sensitivity of interventions in Scotland.

Labour is working with great care because Balls is the party's first shadow chancellor or chancellor since 1987 – apart from Alan Johnson who held the shadow role for three months – not to be a Scottish MP. But there are no Labour nerves about the credibility behind the message to be delivered by Balls, who has warned that the eurozone crisis has shown that a monetary union is fundamentally flawed without a fiscal union. Balls is generally credited with being one of the most influential voices in keeping Britain out of the euro, partly on those grounds.

The three parties believe Carney created the conditions for them to issue their warning when he said a currency union would involve the loss of some sovereignty. However, Carney added that the decision was entirely a matter for politicians in Edinburgh and London.

Osborne told the House of Lords economics affairs committee last week: "I think that, frankly, Alex Salmond's claims about the currency were pretty effectively demolished by the governor of the Bank of England. It was a very non-partisan technical speech from a Canadian citizen, who pointed out that the conditions for a successful monetary union require the ceding of sovereignty and the creation of a banking union. That is why I said last year in Glasgow that I thought it was unlikely that a workable monetary union could be created. I will have more to say on the subject over the coming weeks."

Balls made clear his unease about a currency union in East Ayrshire last month. "Alex Salmond cannot continue to casually claim that Scotland could keep the pound," he said. "He simply cannot make that promise because it would take both sides to agree. The only way to guarantee the pound is to stay in the UK."

The anti-independence campaigners believe they are on strong ground, feeling that Salmond's call for a currency union is the strongest example of his strategy of "de-risking" independence. They also say that his position is inconsistent. In 2009, he told a Catalan television station that there was a strong case for joining the euro in the wake of the financial crisis. He added: "Sterling is sinking like a stone".

One campaigner who opposes independence said: "There are two entrenched camps – around 35% who support the union and will not be budged and around 25% who support independence. Then there is 35% in the middle who are probably open to independence but need absolute certainty that there will be do danger, mainly economic."