The new £20 charge to use the Government's child maintenance service could leave separated parents without any support arrangements in place for their children, an influential committee of MPs has warned.
The Public Accounts Committee (PAC) said there was worrying uncertainty about the impact of the £20 fee hitting low-income families that want to use the statutory child maintenance service.
The 2.5 million separated British families have several options for arranging child support, with 1.1 million relying on the statutory scheme which assesses, collects and makes payments.
The introduction of the fee was intended to encourage more parents to make their own arrangements for child support but PAC chair Margaret Hodge warned that the charge represents a "significant barrier" to poorer families.
She also questioned whether the fee had been set at the right level as research by the charity Gingerbread showed that the number of parents intending to choose family-based arrangements fell from 5,540 in August 2013 to 3,590 in March 2014, after being informed of all the options.
Labour MP Ms Hodge said: "T here is a worrying uncertainty around the impact that the introduction of charges for statutory child maintenance services will have.
"The Department for Work and Pensions' (DWP) primary aim is to encourage more parents to make family-based arrangements rather than rely on the statutory scheme.
"It has an ambitious expectation of reducing the number of statutory cases by 250,000 - one quarter - by 2018-19.
"It is unclear, though, whether charges have been set at the right level to achieve this objective.
"In evidence to us Gingerbread told us it believes the department had been unrealistic about the number of parents who would be able to reach their own private arrangements that last."
She went on: "Research suggests that for parents on low incomes the £20 application fee would be a significant barrier to applying to the statutory service.
"There is a risk that some parents end up reaching no arrangement at all, to the detriment of the children involved.
"The department must monitor closely the number of parents that choose family-based arrangements following the introduction of charging for the statutory scheme."
Ms Hodge called for the DWP to monitor how many people are switching to family-based arrangements since the June 2014 introduction of the charge.
Gingerbread, a charity for single parents, backed Ms Hodge's call for monitoring but also urged the Government to drop the charge.
The charity's chief executive Fiona Weir said: "At the moment, only two-fifths of the UK's two million single parent families receive child maintenance from their child's other parent.
"Since a £20 application charge to use the new service began this summer we have already seen a dramatic fall in the number of parents applying for help.
"At a time when every penny counts for families, £20 is more than many single parents can afford.
"We warned the government that the charge could see the numbers of children getting child maintenance fall.
"We don't know yet whether the parents put off by the fee are going on to make arrangements on their own or are just giving up. The figures we've seen so far seem to confirm our fears.
"The Government should not be putting barriers in the way of the three million children growing up in single-parent families getting the support they need and we urge the DWP to drop the charge."
Child Maintenance Minister Steve Webb said: " This report clearly recognises the progress we've made in making child maintenance simpler. The old CSA system was massively costly yet saw thousands of children get no regular financial support. The new service we're introducing provides better support for parents and gets more money to more children.
"The Government wants to help parents reduce conflict after a separation and support them to co-operate. The charges are there to encourage parents to work together for the good of their children, rather than just rely on the state by default.
"Already the majority of parents using the new Child Maintenance Service are choosing to pay each other directly, thereby avoiding ongoing collection charges.
"We will, of course, continue to monitor closely the take-up of family-based arrangements under the new system."