A war of words has broken out over deficit reduction with Labour and the Conservatives accusing each other of shifting position on the key General Election battleground.

A new Office for Budget Responsibility charter issued by the Treasury sets the goal of eradicating the structural current deficit on a rolling three-year horizon, which at the time of the next Budget will be 2017/18. Previously, the target had been to balance the books on that measure over a five-year horizon.

The charter also specifies debt should be falling as a percentage of GDP by 2016/17; a year later than planned when the Coalition came to power.

The commitments are in line with Liberal Democrat policy but stop short of enshrining the Conservative proposal of achieving an absolute budget surplus - covering current and capital expenditure - by 2018/19.

Ed Balls said Labour would support the new charter in a Commons vote, due in the New Year.

But he accused the Chancellor of a u-turn, saying the March Budget had indicated there would be a tougher mandate on the deficit.

"Once again, a silly political stunt by George Osborne has totally backfired," declared the Shadow Chancellor.

"In the Budget, George Osborne talked about a vote on balancing the overall budget. Today, he and David Cameron have done a staggering u-turn on this vote and are now proposing a vote on the current Budget, excluding capital investment.

"This is the same measure of the deficit the Labour Party has been committed to targeting for the last three years," he explained. "They have also changed the fiscal mandate from being a 'target' to an 'aim'. This charter is consistent with our position, so we'll vote for it," Mr Balls made clear.

But the Tories hit back, insisting the Shadow Chancellor had changed Labour's approach by accepting the 2017/18 rolling target and accepting £30bn more in cuts were now needed.

"Labour are in chaos on the deficit. They can't decide what they should target or when," said a Conservative Party spokesman.

"That's why it's clearer than ever that Labour have no plan to deal with the debts that Ed Miliband and Ed Balls left behind."

Earlier, the Prime Minister warned voters Labour's policies could add £500bn to the national debt over the long term; the equivalent of £30,000 for every taxpaying household in Britain.

He said the public faced a choice in May between "competence and chaos" and urged them to "stick with" the Tories' "sensible and reasonable" plan and reject Labour's "reckless" alternative.

"Ed Miliband is saying he will only balance part of the government's budget - not the whole thing like we plan," Mr Cameron declared.

"Let's be clear what that means: Ed Miliband would never clear the overall, headline budget deficit; he would run a budget deficit - permanently adding to debt - indefinitely, every year, forever."

Meantime, Danny Alexander for the Liberal Democrats said: "Ed Balls and Ed Miliband have spent four and a half years marching the Labour Party up the hill of deficit denial; now they face the humiliation of having to march them back down. Their economic credibility is in tatters."