Nick Clegg has outlined plans to press ahead in April with changes to allow couples to share maternity leave.
The new scheme, first proposed by the last government, will let a father take up any remaining unpaid leave if their partner goes back to work early.
The deputy prime minister said current paternity rules were "Edwardian".
He also said the government would soon be "saying more" about plans to extend the right to ask for flexible hours to grandparents and close family friends.
In a speech to the Demos think-tank, he said the government would consult on a "proper" system of shared parental leave to be introduced in 2015.
He said it was vital that reforms to parental leave helped not only the rich, but also ordinary working families in what he calls "alarm clock Britain".
"Too many of these parents feel trapped by the current rigid rules," he said. Rules allowing men "a paltry" two weeks of paternity leave, "patronise women and marginalise men", Mr Clegg added.
Changes due in April mean that if a mother returns to work without taking a full year's maternity leave the father will be able to take the remaining time, up to a maximum of six months.
Mr Clegg said: "But we want to go further. We know that men need to be actively encouraged to take time off. And often parents want more flexibility than these arrangements will allow.
"So in the coming weeks we will be launching a consultation on a new properly flexible system of shared parental leave, that we aim to introduce in 2015. I would have liked it to be sooner. But getting this right will take time. "
'Use it or lose it'
He said it would have "massive consequences for parents" which had to be carefully considered. "It would be wholly irresponsible to rush these changes... we cannot just spring them on employers."
One idea being looked at was allowing parents to share the overall leave allowance - paid and unpaid - between them in one big block or in lots of chunks.
"Crucially, we're also looking at what can be done to encourage men to take more leave. Possibly, for example, through use-it-or-lose it blocks of time, especially reserved for fathers," said Mr Clegg, who took a long period of leave after the birth of his first child so his lawyer wife could return to work first.
He said any changes had to "reflect the difficult economic circumstances we find ourselves in" but it was right to examine them through consultation.
Mr Clegg also said the government hoped to extend flexible working beyond mothers and fathers "if we are to dispel the stigma many men, and some employers, still attach to it".
"By extending flexible leave, for example to grandparents, or close family friends, we hope to make it much more common - a cultural norm."
For Labour, shadow equalities minister Yvette Cooper said it was important parents were supported in balancing work and family life and that Labour had increased and extended maternity leave and the right to request flexible working.
"But Nick Clegg's attempt to pretend this is a family friendly government too, is a sham. David Cameron has already slammed extending paternity leave as 'political correctness.' And they are cutting £1,200 from families with babies," she said.
TUC general secretary Brendan Barber said: "The UK has the most unequal parental leave arrangements in Europe so extending the previous government's shared leave plans is welcome.
"New parents should be able to decide for themselves who looks after their baby in the first year, rather than having the decision dictated by government regulation, as is currently the case."
However David Frost of the British Chambers of Commerce, warned that the proposals would simply burden business with more red tape and could deter firms from taking on new staff.
"Business is not against the principle of shared parental leave, but how is an employer expected to plan and arrange cover with this fully flexible system?" he said.
Katja Hall, from the CBI employers' group, supported more flexible parental leave but added: "Allowing parents to take ad hoc parcels of leave would make it hard for companies to plan ahead, so we favour parents taking bigger blocks of leave in one go... Any changes will need to be simple to administer and must allow firms to plan ahead to cover staff absences."