New fathers will be able to take up to 12 months of parental leave, Nick Clegg has announced - but affordability constraints could prevent 95% of them from doing so.
The TUC cited the government's own estimates on potential takeup of the full parental leave offer, which found just one in 20 fathers would be able to afford it if it is paid at the current statutory rate of £137 a week.
Unions have nevertheless welcomed the switch towards a more equitable system for caring for infants as a step in the right direction.
"Shared parental leave is a welcome new step that should encourage more fathers to get involved in childcare from the very beginning," TUC general secretary Frances O'Grady said.
"But unless it is backed up with better pay many couples simply won't be able to afford to take it."
The coalition said it would "look at extending paid paternity leave and pay once the economy is in a stronger position".
Under present rules fathers are only permitted two weeks of paternity leave. From April 2015 that will change, with either parent allowed to take 52 weeks of parental leave currently only available to the mother - minus the two-week post-natal recovery period the mother is expected to take.
"We need to challenge the old-fashioned assumption that women will always be the parent that stays at home – many fathers want that option too," the deputy prime minister said.
"That is why from April 2015 we're introducing shared parental leave to allow couples to make that decision jointly ensuring all career options remain open to women after pregnancy."
Existing rules specify that maternity leave must be taken in one continuous block. While parents will get the opportunity to configure their leave in any way, the coalition is giving employers a veto over any proposal to take leave in a discontinued block.
Nicholas Robertson, head of employment at international law firm Mayer Brown, commented: "There is cost and complexity for employers, and the rules have to be clear and easy to apply.
"It is essential that both employers affected and both parents involved know what is fair, and what their rights and obligations are."
Fathers will also be given the right to unpaid leave in order for them to attend up to two ante-natal appointments.
The right to request flexible working after 26 weeks of continuous service with an employer will be extended to everyone, rather than just those who are parents of children under 17 or carers of adults.
The right to return to the same job, which currently applies for those returning to work before 26 weeks, will not be extended, however.
"By failing to give parents the right to return to the same job after six months, the government has also missed an opportunity to prevent a constant source of pregnancy discrimination, where mums returning to work find that their job has changed," O'Grady added.
"The proposal will create confusion for employers and be a source of anxiety for women who decide to take more than six months of pregnancy leave."
Under Clegg's plan employees will have the right to return to the same job even if the 26 weeks are taken in discontinuous blocks.
"There shouldn't be a one-size-fits-all approach – that's not how families are set up," he added.
"Many businesses already recognise how productive and motivated employees are when they’re given the opportunity to work flexibly, helping them retain talent and boost their competitive edge. This is good for families, good for business and good for our economy."