The Conservatives have been accused of using “dishonesty as a weapon” after making inaccurate claims that the Government had halved Britain’s deficit.

The claim – contained in the party’s first general election campaign poster – was branded a “fib” and condemned as deceptive even by right-leaning commentators.

What was intended to be the smooth launch of the new year Conservative message on the economy rapidly unravelled just hours after David Cameron had unveiled the poster which is due to go up on billboards across the country.

Under the headline “Let’s stay on the road to a stronger economy”, the poster goes on to list the Government’s achievements of “1.75 million more people in work”, “760,000 more businesses” and “the deficit halved”.

But the final claim was immediately debunked by the normally supportive Spectator magazine.

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It pointed out that the assertion was only true if the deficit – the difference between what the public sector receives and what it spends each year – is measured as a proportion of national income. In cash terms, the deficit has only been reduced by about a third.

Writing on the Spectator’s website the magazine’s editor and former business journalist Fraser Nelson said: “It’s a relatively small deception ... but this poster makes a more important, and more depressing point: the Tory leadership is prepared to use dishonesty as a weapon in this election campaign.”

Mr Nelson said that most people understood the word “deficit” to mean the amount, in cash, between what is spent, and what is raised.

But the Tories claim that the deficit had been halved was based on saying the word “deficit” has another meaning: the ratio of deficit to GDP.

“I don’t dispute that the ratio is widely accepted and more useful to economists, but a deficit/GDP ratio is different to ‘the deficit’ which is measured in pounds,” Mr Nelson wrote. “If you want to talk about the ratio, you need to say so – otherwise the sentence is a porkie.”

Labour joined in the attack with the shadow minister Chris Bryant accusing the Conservatives of skewing the truth.

“It is a bit troubling when [the] first Tory campaign poster has a fib,” he said.

On Twitter the Tories attempted to rebut Mr Nelson’s assertion tweeting: “You are simply wrong about this. The independent Office for Budget Responsibility said at (the) Autumn Statement “the budget deficit has been halved to date.”

But Mr Nelson took on that claim as well – pointing out that the full OBR quote included the caveat that the deficit had only been halved “relative to GDP”.