By Ollie Stone-Lee
BBC News website political reporter
Plans for a 10-hour school day risk encouraging some parents to think the state will bring up their children, says a Lib Dem frontbencher.
Ruth Kelly wants more breakfast clubs as part of the new hours
Work and Pensions spokesman David Laws fears the government plan could undermine family support, with the "nanny state replacing the nanny".
Mr Laws also tells BBC News his party should not challenge a Gordon Brown-led Labour Party from the Left.
And he urges Tony Blair to prevent Mr Brown blocking pensions reform.
Education Secretary Ruth Kelly on Monday unveiled details of plans for schools in England to open from 8am to 6pm.
Mr Laws said in one sense the proposals were the logical outcome of many government policies supported by opposition parties to help people to keep working if they have younger children.
But he highlights the challenges posed by the "massive breakdown of traditional family support structures" over recent decades, including rises in lone parenting and child poverty.
"One of the potential answers is for the state to get involved to try to act in loco parentis, essentially the nanny state replacing nanny," he said.
"Some of that I think is necessary if we are to turn round the consequences of that big social change.
"But I'm nervous in going too far in encouraging parents to think that there is something called the state which will look after their children, bring their children up and substitute for family support."
Mr Laws said the government should not be prescriptive in saying either that parents with young children must go work or that they must stay at home.
Instead, parents should take those decisions, he argues, with the right balance between the duties of the state and private responsibilities.
Mr Laws also fears the school hours plan might be done "on the cheap" and questions whether in some rural schools a head teacher might be left effectively as a childminder to only a few children.
Mr Laws took up the work and pensions brief in the Lib Dems' post-election reshuffle.
He is concentrating on the government-commissioned review of pensions, which is due to report this autumn.
He met the commission's chairman, Adair Turner, last week to discuss his broad approach to the pensions "crisis".
Charles Kennedy says the Lib Dems need a narrative
"I think there is a consensus emerging - outside the government anyway - about what's wrong with the existing pensions system and in particular this means-tested muddle of a system which has been engineered by Gordon Brown in particular since 1997 is unsustainable," said Mr Laws.
There needs to be a simpler, fairer, more sustainable state pension which provides a guarantee against poverty for everybody in society, he argues.
On top of that, extra saving must be encouraged among as many people as possible instead of just helping more affluent people who would put money aside in any case.
Chancellor Gordon Brown has suggested consultation on major pensions changes could continue beyond the next election.
Mr Laws suggests Mr Brown wants to keep his means test based system and wants to keep consulting until he is prime minister and can take the key decisions.
"If the prime minister is serious about getting pensions reform he really needs to do it quickly so it doesn't end up being another one of those issues where Gordon Brown is able essentially to be a roadblock."
Mr Laws said failure to act on pensions reform could mean changes in the pension age being sprung on people at very short notice in 20 or 30 years time.
Raising the pension age would need 25 years notice, says Mr Laws
The Lib Dems have already pledged to give at least 15 years notice of any change to the state pension age - Mr Laws now suggests there should be a 25 year warning.
He also doubts there will be consensus on raising the state pension age beyond 65 in the next couple of decades and wants as much as possible done through individual choice, not political diktat.
An office move has left Mr Laws incommunicado without a telephone - could this be an analogy for the difficulty he may face in calling on his party to embrace more choice and competition?
He laughs and says he has just been countering the centre left's "rather lazy" assumption that competition and choice has to be about Conservatives allowing the wealthy to opt out of state public services.
The Lib Dems are best placed to achieve a balance between social justice and economic liberalism, he argues.
The Lib Dems "should not try to go to the left of Gordon Brown"
While rejecting claims that the Lib Dems have shifted to the Left, he is clear about what would be the wrong direction for his party.
"The idea that we can compete with a Brownite Labour Party from a more centralised, statist perspective and more classically left-wing position I think would be a mistake both in terms of political strategy and in terms of party policies."
Mr Laws said the Lib Dem election manifesto was "pretty true" to the party's Liberal traditions.
"If anybody says to you that they believe in every last dot and comma of party policy in any political party then they're either a liar or very unimaginative because party policy is developing all the time," he said.
With a full policy review now starting, Mr Laws refuses to be drawn on which policies he would like to see dropped.
He denies the Lib Dem proposal to replace council tax with a local income tax was a "liability".
And he says only that it would be strange not to review the party's call for a 50p tax rate on earnings over £100,000 as part of its tax commission given that it is one of its major tax policies.
That should include looking at how Lib Dem proposed tax levels compare with those in other competitor nations, he said.
The Lib Dems made a net loss of seats to the Tories but Mr Laws argues their showing against Labour puts them in a "good" second place to the governing party in a "swathe of seats" which can be won at the next election.
He argues the "narrative" his party must use to explain its vision at the next polls must be Liberalism. What exactly that means will be up for grabs as the Lib Dems wrangle in the policy review in the months ahead.