Unions and a growing number of Labour MPs are concerned that the statutory right to unpaid leave, which will be launched tomorrow, will disadvantage poorer parents whose incomes are already stretched because they cannot afford to take time off without pay.
They are also fear that it could lead to "nursery apartheid" because it will only apply to babies born after tomorrow's cut-off date, which would exclude millions of parents who already have children aged under five. A spokesman from the GMB general workers' union said: "Parental leave should be paid to make sure it is an opportunity not just for the wealthy. The Government should also abolish the cut-off date, which will effectively lead to a form of nursery apartheid because it will discriminate between children born just days apart." He added that the directive should be back-dated to give all parents the right to take the 13 weeks' leave.
Julie Mellor, who chairs the Equal Opportunities Commission, urged Mr Blair in an open letter to use his "unique opportunity to lead by example. Regardless of how much leave you do or don't take decide to take, you are in a position of authority that allows you to say that it is OK for men to talk about their parental responsibilities, and to take time from important jobs to be an responsible father at home," she said.
"You can help to end the artificial divide in British working life, which maintains that family life and working life are entirely separate spheres."
The former social security secretary Harriet Harman has also stressed that the leave should be paid and should be more flexible so it is fair to all. "This is the first step of many towards a great cultural change in society," she added.
But while ministers have privately admitted that there was legitimate concern over the legislation, they have insisted the Government was worried about a backlash by employers if it was extended. "I know that there is a level of unfairness but then that will always happen if there is a cut-off date. We will consider whether it would be realistic to introduce paid leave but we have to be careful that it will not be seen as another burden for business," one ministerial source said.
Under the legislation, parents can take no more than four of the 13 weeks consecutively in any one year between their child's birth and its fifth birthday. Paid maternity leave has been extended from 14 to 18 weeks.