Working parents with children aged three and four would receive 25 hours of free childcare a week under new Labour plans.
Shadow chancellor Ed Balls has pledged to increase the hours covered by state funding from 15 to 25, where a single parent or both parents work.
The move, unveiled in his keynote speech to the Labour party conference in Brighton, comes after another proposal to extend childcare at primary schools from 8am to 6pm.
Under Labour's plans, which it suggests would be funded through an increase in the bank levy, the 15-hour early years entitlement would also remain universal.
Ed Miliband congratulating Ed Balls after his speech
Mr Balls dismissed Tory claims of a £27bn black hole in his economic plans as "nonsense" as he and Ed Miliband battle to restore public trust in Labour on the economy.
And he also appeared to signal a significant weakening in Labour support for HS2 rail link, suggesting the potential £50bn price-tag might be better spent elsewhere.
"The question is - not just whether a new high speed line is a good idea or a bad idea, but whether it is the best way to spend £50bn for the future of our country," he said.
The shadow chancellow has written to the Office for Budget Responsibility (OBR) asking for an audit of his spending commitments but the watchdog cannot go ahead under its current remit.
He has therefore called on parties to unite and push for a change so that both the opposition and Government can be scrutinised independently.
Ed Balls playing football on Sunday
"This is the first time a shadow chancellor - the first time any political party in Britain - has ever said it wants this kind of independent audit," he told delegates.
"It's a radical change from what's gone before, but the right thing to do to help restore trust in politics."
The call came after Treasury analysis commissioned by the Tories suggested Labour promises would require more than £1,000 in extra borrowing per household in 2015.
OBR chairman Robert Chote warned there would be "practical issues" if its remit was altered, with questions about resources and access to the right data.
And Tory Treasury minister Sajid Javid branded it 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."
"Nothing has changed - it's the same old Labour. Ed Balls and Ed Miliband still want more spending, more borrowing and more debt - exactly how they got us into a mess in the first place."
But Mr Balls insisted Labour would show "iron discipline" if it regained power. "There will be no more borrowing for day-to-day spending," he vowed.
He admitted there would be "tough choices" if it does return to Government in 2015 and that it would not be able to reverse all of the coalition's measures.
Growth and jobs "cannot magic the whole deficit away at a stroke" and the coalition's spending totals for 2015/16 would have to be Labour's "starting point", he said.
"Any changes to the current spending plans for that year will be fully funded and set out in advance in our manifesto."
With the conference focusing on the cost of living, Mr Balls argued that the recovery was not translating into any change for struggling British families.
He accused David Cameron and Chancellor George Osborne of condemning Britain to "three wasted and damaging years" and only helping a "privileged few".
He claimed the Government's bank levy has raised £1.6bn less than the coalition promised and that institutions paid £2.7bn less in overall tax in 2011 compared to 2010.
"At a time when resources are tight and families are under pressure that cannot be right," he told delegates.
"So I can announce today the next Labour government will increase the bank levy rate to raise an extra £800m a year.
"And we will use the money, for families where all parents are in work, to increase free childcare places for three and four- years-olds from 15 hours to 25 hours a week.
"For the first time, parents will be able to work part-time without having to worry about the cost of childcare."
However, he confirmed plans to scrap the so-called "bedroom tax" that cuts housing benefit for council tenants with extra space.
A compulsory jobs guarantee for young people and long-term unemployed would be funded via the tax on bank bonuses and cuts to pension tax relief for top earners, he added.
And he vowed Labour would go ahead with a 10p tax rate paid for by a mansion tax on properties worth more than £2m, as well as improvements to the national minimum wage.
Some have suggested that asking the OBR to assess the credibility of Labour polities would just extend what is already in place.
The Prime Minister's spokesman said there was an "established process" allowing ministers to ask Treasury officials to cost opposition ideas.
This was most recently used by Mr Javid and led him to make the claim about a £27.5bn black hole of unfunded Labour plans.
The tool has been used by all sides - Labour asked for 38 Tory policies to be costed before the last election, including moves on inheritance tax and stamp duty.