ED Miliband has sought to seize the initiative on the economy by claiming Labour is now the party of fiscal responsibility as he once again placed it on the side of ordinary workers.

The Labour leader also aimed to portray the Tories as being on the side of the rich and privileged.

But the party leader suffered a blow to his credibility when the respected think-tank, the Institute for Fiscal Studies(IFS), suggested the promises contained in Labour's manifesto were vague, making it difficult for voters to know precisely what they would be voting for if they backed Mr Miliband's party.

Addressing loyal supporters at the old Granada TV Studios in Manchester, the Leader of the Opposition insisted Labour was the party of change and responsibility, which wanted to transform the way Britain was run and who it was run for.

Insisting that he had been tested as Labour leader and he was now ready for power, Mr Miliband, flanked by members of his Shadow Cabinet, said: "Ready to put an end to the tired old idea that as long as we look after the rich and powerful, we will all be OK. Ready to put into practice the truth that it is only when working people succeed that Britain succeeds."

He declared: "If you elect me as your prime minister in just over three weeks' time, I will work for that goal. I will fight for that goal every single day in everything I do, in every decision I make. I know Britain can be better."

The only new policies in the 83-page prospectus for power were promising to bring forward to October 2019 the pledge to raise the minimum wage to £8 an hour and a freeze on rail fares south of the border for a year in cash terms.

But the key aim of the launch was to convince voters that Mr Miliband and Ed Balls, the Shadow Chancellor, could be trusted with the nation's finances; an area where they still trail heavily behind David Cameron and George Osborne in the opinion polls.

The policy document made clear that the current budget deficit would be eliminated and debt would be falling as a percentage of GDP as soon as possible in the next parliament ie by 2020 at the latest.

The party leader stressed how the first page of the manifesto "sets out a vow to protect our nation's finances; a clear commitment that every policy...is paid for without a single penny of extra borrowing".

Mr Miliband said Labour would not promise anything it could not fund, contrasting this with the "fiscally irresponsible" Conservatives, whom he described as the "party of sums that do not add up and commitments that cannot be kept".

He added: "The plan we lay before you is no less ambitious because we live in a time of scarcity. It is more ambitious because it starts from a clear commitment to balance the books and more ambitious because it does not stop there. It meets the scale of the challenges we face today with not one policy funded by extra borrowing."

Pressed in a post-speech Q&A, precisely whenLabour would balance the books on the current deficit, Mr Miliband argued the Lib-Con Coalition had shown it was wrong to set "arbitrary" deadlines for deficit reduction.

But the IFS suggested this left voters in the dark. Paul Johnson, its Director, said: "The big unknown on the Labour side is how fast they want to get there and, therefore, do they need significant spending cuts or tax rises at all."

He explained because Labour had given itself until 2020 to balance the books on the current budget, then it could avoid £18bn of cuts over the next three years.

"It allows them to say: 'Well, we would be cutting very little but also that we would be cutting.' But it really makes a big difference; there's a huge difference between £18bn of cuts over the next three years and no cuts. Literally we would not know what we were voting for if we were going to vote for Labour," he said.

Mr Miliband's opponents were scathing about Labour's claims of fiscal responsibility.

In Cleveland, the Prime Minister said it was "not a conversion to responsibility, it is a con trick" as more borrowing would mean more taxes.

Nick Clegg compared Labour's promise on borrowing to a bottle-a-day alcoholic.

"It's a dangerous addiction. The Labour Party have no plan and no date by which to clear the decks, wipe the slate clean and deal with the deficit," added the Liberal Democrat leader.