The Department of Health has refused to release correspondence to Monitor regarding its legal dispute over the cap on income from private patients.
The DH submitted a response to Monitor after it revealed in December that Unison had launched judicial review proceedings against its interpretation of the cap.
HSJ asked the department for a copy of that response through the Freedom of Information Act, but the DH decided not to release it, saying it would be prejudicial to the "effective conduct of public affairs".
A letter to HSJ said it was important that Monitor and the DH maintained a "close working relationship and good communication at all levels" and that disclosure could "undermine that relationship". There was a risk that Monitor would be "less inclined to engage in written or oral discussion with [the DH] if communications between the organisations became a matter of public record."