Fathers are being deterred from taking a bigger role in caring for their children because of lost earnings to the family budget, according to a new report.
A survey of 1,500 workers by think tank Demos showed that only one in 10 men would use a longer period than the current two weeks of paternity leave.
Only half of men take their full entitlement, often because statutory paternity pay covers less than a quarter of their salary.
The study found that under the new transferable leave arrangements, average-waged fathers who took six months' leave stand to lose a huge proportion of their earnings.
The report said this was preventing men playing a more prominent role at home after their child is born and could act as a barrier to getting mothers back into work following pregnancy.
"Flexible working is the only option for Britain to address the social challenges of shared parenting and an ageing population that requires care. It will also be crucial to the success of the Government's Big Society agenda," said the report's author Dan Leighton.
"As it stands, parental leave is expensive for the employee, the employer and the state."