Any potential merger would be subject to a public consultation

Conwy and Denbighshire and North East Wales NHS Trusts will conduct a six-week feasibility study, but a decision will not be made until 2008.

The trusts said they would not speculate on any potential impact until the study was completed.

But health workers' union Unison said it was "bemused", and argued that mergers did not necessarily benefit staff or patients.

The proposal follows an announcement in July by Health Minister Edwina Hart that seven health trusts in Wales had been cleared to discuss possible mergers.

We are sceptical about the benefits to staff - size is not everything Dave Galligan, Unison

She said the aim was to cut bureaucracy and deliver better care for patients.

Ms Hart has already approved the merger of Pontypridd and Rhondda and North Glamorgan NHS Trusts by 1 April 2008.

The feasibility study in north Wales was announced by trust chief executives Gren Kershaw and Mary Burrows.

In a statement, they said: "The pace of technological change and increased specialisation, particular in hospital care, clearly points to closer working between NHS Trusts.

'Best possible service'

"The recent north Wales secondary care review which involved doctors, nurses, professional staff and patient representatives said that we need to look at how we manage these changes, to ensure that patients get the best possible service."

The study's findings will be reported back to the trust boards in November, and would be put out to public consultation before any merger was agreed.

Dave Galligan, Unison's head of health in Wales, said: "There seems to be merger mania at the moment.

"Our experience is that there are rarely benefits to staff from mergers.

"We are sceptical about the benefits to staff - size is not everything."

He said he also feared patients would not benefit, adding: "The bigger a trust gets, the more remote and inaccessible it tends to become."