Campaigners calling for an end to mixed-sex wards in psychiatric units have attacked advice on new builds issued by the NHS Executive.

Redbridge Healthcare trust in Essex wrote to health secretary Frank Dobson for guidance after he had halted the construction of a psychiatric unit at Charing Cross Hospital in May.

The decision followed claims that the new unit contravened the government's determination to get rid of mixed-sex wards.

Redbridge Healthcare trust had almost completed plans for a 7m unit when the furore broke.

It received a two-paragraph reply from Judith Guest of the Executive's Patient's Charter unit, which said sleeping and washing areas should be separate and include 'suitable arrangements' to ensure safety.

'But completely segregating services would not be acceptable, as there are good clinical reasons to provide an environment which, as closely as it can, prepares people for a return to their own accommodation,' she said.

Redbridge Healthcare trust finance director Ian Cable said clinicians and managers were happy with the response, even though it meant some 'major rethinking' to 'enlarge the whole envelope to incorporate segregated bathing and sleeping arrangements.'

But campaigners demanding complete segregation in acute units were angry that a message from the Executive seemed at odds with Mr Dobson's earlier view.

Julian Lewis, the New Forest Conservative MP who first raised the issue of Charing Cross in the Commons, wrote to Mr Dobson complaining that the letter showed the Executive was 'backsliding'.

Grainne McMorrow, a solicitor working for mental health charity SANE, said the letter 'gave no clear guidance at all' and illustrated the 'continuing confusion' over best policy in acute units,

'It is an astounding cover-your-arse statement,' she said, adding that the Executive should 'at least' have asked to see the plans .

Architect and mental health planner Neil MacDougal, who served on the working party that produced the influential Not Just Bricks and Mortar report on mental health design, said it was an 'insult' that the letter had come from the Patient's Charter unit.

But a Department of Health spokeswoman said there was no confusion.

'Throughout the NHS there are places where the sexes need to be segregated to keep patients safe, but there are developmental reasons why hospitals should include some integration of the sexes under close supervision,' she said.

'It is not the case that there should be complete segregation across the board.'