ED Miliband will today warn his party and voters that Labour will have to stick to the "hard reality" of austerity and cannot now promise to reverse any of the £11.5 billion of extra cuts for 2015/16, which the Chancell or will set out in a spending review on Wednesday.

The Labour leader will say the party will have "iron discipline" on the economy if it is elected at the 2015 General Election..

He will insist his government would not reverse the Coalition's current spending cuts unless other cuts could be found from elsewhere or taxes were raised. He will insist Labour would not borrow any more money to make up the difference.

Mr Miliband's speech to Labour's National Policy Forum in Birmingham comes as No 10 said George Osborne's 2015/16 savings plans were "on track" and Treasury sources indicated "we're almost there". It is thought only Vince Cable's Business Department is holding out against an extra 10% cut in its spending.

The Labour leadership, which has already sought to convince the public of a tougher approach to spending – pledging to scrap the winter fuel allowance for richer pensioners and making clear it would not reverse the Coalition's child benefit cuts for the better off – knows it is only likely to regain power if voters believe it is hard-headed and disciplined on the economy.

Mr Miliband will tell delegates a new Labour government would gain power in "tougher economic circumstances than we have seen in generations and that will have to shape the way we govern".

He will make clear the party's starting point for 2015/16 would be not to reverse any cut in day-to-day current spending unless it could be fully funded from cuts elsewhere – meaning the overall total would remain the same – or from extra revenue such as tax hikes. More borrowing would not be an option.

Mr Miliband will say: "So when George Osborne stands up next week and announces his cuts in day-to-day spending, we won't be able to promise now to reverse them because we can only do so when we can be absolutely crystal clear about where the money is coming from. It's a hard reality. But I am clear about it, Ed Balls is clear about it and everyone in the Labour Party should be clear about it too."

He will insist: "People will only put their hope in us if we show how we will make a difference. But people will only put their trust in us if we show we are credible. Only if we have the discipline to face the challenge of our times, can we change the direction of our country."

Meanwhile, the Chancellor is said to be "almost there" with completing his bid to find an extra £11.5bn in savings for 2015/16.

Already, ministers have said 13 out of 23 Whitehall departments have settled, including the Home Office and the Ministry of Defence. But it is thought the other big spender, the Business Department, has been holding out against a 10% cut. Treasury sources appeared confident Mr Cable would fall into line by Wednesday.