Tory plans for a new budget watchdog are overly bureaucratic, other opposition parties have claimed.

The Liberal Democrats and the Green Party said the Conservative proposal for an Office for Budget Responsibility, unveiled by shadow chancellor George Osborne at the party’s conference on Monday, was unnecessary.

Lib Dem Treasury spokesman Vince Cable said his party had long advocated the introduction of a “genuinely independent” system for monitoring budget management. “The Tories seem to have found a rather complicated way of doing the same thing,” he said.

“It should not require the creation of a new bureaucracy, as the Tories want," he added. “The job could easily be done by the National Audit Office (NAO), whose independence is recognised by all.”

Green party leader Caroline Lucas took a similar stance, claiming that the move would outsource oversight of government finances to a quango when parliament already did the job.

“We don't need another quango, filled with the same corporate bosses that got us into this mess, with a few Tory donors and ultra-right think-tank wonks for good measure,” she said in a statement following Osborne’s speech.

On the third day of the conference, Conservative shadow chancellor said the Tories’ first priority would “have to be bringing stability to the public finances”.

They would scrap the fiscal rules that had been “fixed and fiddled so many times by Gordon Brown no one believes them any more”, he promised.

The Tories would then put government finances “on a path out of the red and into the black”.

He continued: “But I want to make sure that this won't be yet another promise made by a politician and which is then broken. So we will do what no government has ever done before.

“We will set up a new Office for Budget Responsibility, independent of government like the Bank of England is.

“This independent office will stand in judgement over our commitments, hold us to our promises. And it will let the whole country know if we try to wriggle out of them.

“No more fiddling of the rules. No more dodgy statistics. No more hidden PFI borrowing,” he ended. “Chancellors will be forced to behave responsibly with the public finances or face the public consequences.”

After his speech, a Tory spokesman clarified the Tory's stance, insisting that there was a need for a new office, with its fiscal forecasts and oversight of overall government spending.

"This is not an attack on the NAO or the Office of National Statistics," he explained. "They don't have a mandate to do what the new OBR would do."