In a joint statement, the children's charities say the agency will do little to alleviate the hardship faced by families on income support and may even make it worse.
Their comments follow criticism of the new organisation from a broad spectrum of groups, ranging from feminists to campaigners for men's rights.
All agree, however, that the agency represents one of the most profound changes in family law of the last 20 years. Until now, maintenance for children has been set by the courts, with single parents expected to ensure that they receive any money due to them. The system has been criticised as inconsistent and unfair: less than 30 per cent of single mothers obtain any child maintenance at all from their former partners. Those that do often get sums described as 'derisory'.
From today, the agency will take over responsibility for assessing and collecting maintenance. Under the new system, payments will double to an average of pounds 48 a week and the proportion of lone parents receiving maintenance is expected to rise to 50 per cent. The figures will be worked out on the basis of a set formula, ensuring consistency, according to Ros Hepplewhite, the agency's chief executive and former director of Mind, the mental health charity.
Virtually all the 895,000 single parents who receive Income Support will be forced to have their cases assessed by the agency, even where they are happy with the arrangements made with ex-partners. Those not receiving benefit can chose whether they use the new organisation.
Ms Hepplewhite says the agency will establish an effective system for obtaining money from absent fathers. Even homosexuals who have donated sperm to enable lesbian couples to have children could find themselves liable to pay maintenance. Only men who have donated through registered clinics will be exempt, the agency said last week.
But the five children's charities point out that if women receive increased maintenance, their income support will be reduced proportionately, leaving them no better off.
Indeed, the charities say that some women will lose their entitlement to state benefit, which will also mean that they lose the right to free milk, school meals and prescriptions. Further, these women will be dependent on money from an ex-partner who may be less than willing to pay every week. Hardship could be the result, the charities say.
There is also fierce criticism of a clause in the Child Support Act compelling women to name the father of their children. If they refuse do so without good reason - such as the threat of violence - 20 per cent of their benefit can be deducted. Ian Sparks, director of the Children's Society, said this would expose many lone parents and their children 'to greater risk of violence, harassment and poverty'.
Organisations representing men also have criticisms. They are particularly aggrieved that so-called clean-break settlements, under which wives keep the family home in return for reduced maintenance, can be declared null and void. This is at best anomalous, at worst iniquitous, they say.
Last week, a spokesperson for the agency accepted that only 'a few thousand' single mothers would be financially better off a result of the new system. In contrast, the Government hoped to save pounds 530m in the first year year.
But he said that increased maintenance would give single parents the security of knowing that they would continue to receive money if they returned to work. Those on income support do not have that option, he said.
Speaking on BBC Radio 4 yesterday, Peter Lilley, Secretary of State for Social Security, defended the clause allowing officials to dock benefit. 'We will be operating on the presumption that a mother's word is to be trusted unless it is contradicted or manifestly unbelieveable,' he said.
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