Ministers wrestling with schemes to resolve the crisis at the CSA were alarmed to discover that it was not a criminal offence to give misleading information to CSA officers. The omission by the last Tory government - in spite of tough rhetoric by Baroness Thatcher about the need to make fathers support their children - is seen as a glaring gap in the legislation, which will have to be plugged in the next session of Parliament.
"We couldn't believe it when we found out," a ministerial source said. Ministers are deeply worried that announcing the lack of a penalty for misleading the CSA could be exploited by fathers who are being harassed to make payments for their children.
They plan to introduce measures to deal with the self-employed - because ministers believe they are the most likely to cheat the system - including cross-referencing income tax returns to the Inland Revenue to find out how much the fathers are really earning.
The Secretary of State for Social Security, Alistair Darling, is due to announce the action as part of the Government's strategy for streamlining the work of the CSA following widespread criticism from families that it is cumbersome and unfair.
Mr Darling is expected to announce the go-ahead for the simplified scheme proposed in the government Green Paper, Children First, which will replace a complicated formula for maintenance with easy-to-follow tables showing how much fathers need to pay.
Most non-resident parents with net earnings of more than pounds 200 a week will have to pay 15 per cent of their weekly income for one child, rising to 20 per cent for two children and 25 per cent for three or more.
Baroness Hollis, the minister for social security in the Lords, who was in charge of the review, is understood to have been told by the Prime Minister to come up with more radical plans when she reported to Downing Street that the Green Paper plan should be endorsed.
No 10 is looking for a root-and-branch reform of the CSA in the long run, which could leave the agency under sentence to be privatised, if it failed to improve its performance under the simplified scheme. Ministers had considered totally abolishing the CSA.
Doing away with the CSA would have proved popular with MPs who are having to cope with protest letters about its operation, but ministers feared it could make matters worse. The latter have decided that the simplified system may be "rough justice" but it will be fairer than the complex maintenance formula which has led to backlogs and protests from many second families who feel they are being penalised to pay for the children of an earlier marriage.
David Willetts, the Tory spokesman on social security, said: "They have had more than two years to deal with it. Having promised action at the general election, it is about time they came forward with substantive proposals."Reuse content