Trade union Unison is to push ahead with its legal challenge to foundation trust regulator Monitor over its interpretation of the private patient income cap.

The move could see Monitor's top team appearing before a judge to defend its interpretation of the Health Act 2003, which stipulated that the proportion of income a foundation could earn from private patients should be capped at its 2002-03 level.

Last month Monitor admitted its interpretation of the law had been "too permissive" and said it would tighten its rules from April to make foundations declare private income gained through joint ventures and arm's-length companies.

Judicial review

Monitor had hoped the partial climbdown would be enough to get Unison to drop its case - which the Department of Health's legal advisers have said is watertight. But Unison will today tell Monitor it is proceeding with the judicial review.

Unison head of health Karen Jennings told HSJ: "The cap was a key decider for many MPs who had severe reservations about voting foundation trusts into existence. [It] was put there to ensure that NHS patients were not pushed to the end of the waiting list and treated as the poor relations to paying patients."

Monitor has made no secret of its talks with the DH over changing the law to remove or substantially dilute the cap. Such a change could be easier to get through a Conservative-led Parliament, but Unison will hope to garner public opposition for such a change through its legal case.

Foundations declared private patient earnings of£165m last financial year, but an interpretation as sought by Unison could increase that by 70 per cent to£281m.