Labour will on Monday try to bolster its economic credibility by asking the government's spending watchdog to audit all its tax and spending commitments before the next election.

The shadow chancellor, Ed Balls, will tell Labour's conference he has written to the Office for Budget Responsibility (OBR) to ask it to examine the credibility and implications of the party's manifesto plans. His keynote speech will contain fresh proposals to target help for child care, but also a warning that the economic backdrop means a "Labour government will find delivering its goals harder than at any point in living memory".

The Balls plan to throw Labour's five-year tax and spending plans to the mercy of the judgment of the fiercely independent OBR is high risk, but betrays the struggle Labour is facing to persuade the electorate that it will not be fiscally irresponsible in government.

The Conservatives issued claims that Labour has already made an additional £27.9bn in spending commitments for 2015-16 since June.

The OBR was established by the coalition after claims the Treasury under the chancellorship of Gordon Brown had manipulated fiscal forecasts to make sure his spending plans looked sustainable. Established by statute, its formal remit does not to extend to examining the fiscal plans of the opposition, and it would require the agreement of the coalition itself for the remit to be extended.

Balls wrote to Robert Chote, the director of the OBR, asking the watchdog to investigate Labour's spending plans. Chote said it would be for parliament to decide whether to extend the OBR's remit as set out in its charter, adding: "It would be highly desirable there should be cross-party consensus on that." The charter makes clear the OBR should not be drawn into party politics.

He added the OBR would need extra resources and "a guaranteed access to the necessary data and analytical expertise within Whitehall".

The OBR might be reluctant to be dragged into the bear pit of an election to arbitrate on the credibility of the rival parties' spending plans. But Balls will tell delegates at the Brighton conference: "It is a radical change from what's gone before, but the right thing to do to help restore trust in politics."

The party's leadership is keen to move the news agenda away from the confessional memoirs of Brown's former spin doctor Damian McBride. Ed Miliband, also a former Brown aide, said on Sunday that he had urged Brown to sack McBride. Miliband said McBride's book, serialised in the Daily Mail, served as "a reminder that we must have no factions and no briefing in the Labour party".

Balls will also give his conference a stern economic message, saying the Conservatives' economic failure will mean an incoming Labour government will have to finish the job of eradicating a £90bn deficit.

He will say: "We stand to inherit a very difficult situation. After three wasted years of lost growth, far from balancing the books, in 2015 there is now set to be a deficit of over £90bn … We won't be able to reverse all the spending cuts and tax rises the Tories have pushed through. And we will have to govern with less money around. The next Labour government will have to make cuts, too. Because while jobs and growth are vital to getting the deficit down – something this government has never understood – they cannot magic the whole deficit away at a stroke."

Labour have already made a commitment to match the coalition's current spending plans in 2015-16, but might increase capital spending and have not yet spelled out the pace at which they would cut the deficit.

Insisting he needs to be straight with the British people, Balls will say: "Any changes to the current spending plans for that year will be fully funded and set out in advance in our manifesto. There will be no more borrowing for day-to-day spending. And we will set out tough fiscal rules – to balance the current budget and get the national debt on a downward path."

Balls will also attack the Tories' economic record, arguing: "When David Cameron and George Osborne say that everything that is happening in the economy is down to them, let us remind them: prices rising faster than wages; three years of flatlining; the slowest recovery for over 100 years; a million young people out of work; welfare spending soaring; more borrowing to pay for their economic failure. That is their economic record. And we will not let them forget it."

He will add "For millions of families this is no recovery at all."

Labour will also reveal on Monday leading employers have agreed to participate in its job guarantee scheme designed to give youngsters a job after they have been out of work for more than a year.