Shadow chancellor will also tell Labour conference he wants watchdog to audit all of party's tax and spending commitments

Ed Balls, the shadow chancellor, is to propose extending free childcare from 15 to 25 hours a week, saying he can fund the measure by raising the bank levy.

His offer will be available to working parents of three- to four-year-olds.

It will be available to single parents and to households where both parents are in work.

The move is separate to Labour's pledge at the start of its autumn conference in Brighton to provide "wraparound" care through schools to help ease the childcare burden for families. No money was set aside by Labour for its wraparound offer.

Balls will make the childcare offer in his speech to the party conference in Brighton on Monday, in which he will call for iron discipline on spending.

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

He 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 childcare, 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 put Labour's five-year tax and spending plans at the mercy 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 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.

The Conservatives denounced the plan as a Labour stunt, saying the party has been found out for making unfunded commitments.

The Conservative Treasury spokesman, Sajid Javid, on Sunday released an analysis by Treasury officials according to which, he said, Labour's promises would require more than £1,000 extra borrowing for each household in 2015.

Javid said: "Ed Balls knows this is not allowed under the Budget Responsibility Act, and the OBR's charter, so this is just a stunt to try and distract attention from the fact that Labour have been found out for making unfunded commitments that would just mean more borrowing and more debt."

Speaking on BBC Radio 4 on Monday, Balls said it would not require a change in the law to widen the remit of the OBR but instead just a single word-change in its charter. He lambasted the Tory opposition to his ideas, saying: "They are playing party politics when we are trying to give the public reassurance that all parties manifestos add up to rebuild trust".

He said he did not think the Conservatives would ultimately block the plan, claiming the Treasury select committee chairman, Andrew Tyrie, supported the idea and commented it would not require legislation.

In his speech, Balls will say: "Childcare is a vital part of our economic infrastructure that, alongside family support and flexible working, should give parents the choice to stay at home with their children when they are very small and to balance work and family as they grow older."

"But for many families, high childcare costs mean that it doesn't even add up to go to work. So to make work pay for families, we must act."

He will justify an increase in the bank levy to raise an extra £800m a year to fund the extension.

He argues that in the last financial year, the banks paid £2.7bn less in overall tax than they did in 2010. Over the past two years, the government's bank levy has raised £1.6bn less than ministers said it would.

The current free entitlement, which the government increased from 12 to 15 hours a week in November 2010, has high take-up, although there is some evidence that the most disadvantaged people are not using their entitlement.