The centre has the latest radiotherapy equipment

The discovery, in one of three machines, means there is an increased risk of cancer returning, but the risk is only slight.

A total of 43 patients are being recalled as a result.

It is thought a metal plate inside the unit had moved slightly some time between last August and 8 June this year.

The fault meant that some patients received a reduced dose of radiotherapy in the area requiring treatment.

The hospital reviewed case notes for 135 patients and contacted 43 who are potentially "at risk" by letter and telephone on Tuesday.

The notes will be reviewed again by an independent expert.

In a letter to GPs, the hospital said there was a slight shadow on the edge of the radiation beam.

Not life-threatening

The incident is not thought to be life-threatening. Hospital chief executive Gren Kershaw stressed that a small number of patients were affected and said the risk was very low.

He said he was sorry patients had to be recalled and said the fault had been picked up because of good quality practices at the centre.

The �20m centre opened its doors last June with an array of hi-tech equipment and was hailed as providing an improved service for patients who had to travel to the north-west of England for treatment.

At a cost of �2m, the centre installed two of the latest linear accelerators to provide radiotherapy treatment, but predicted high demand would force them to buy a third within two years.

The centre is at Glan Clwyd Hospital

When the equipment was installed at the centre in September of 1999, officials said it was so complex that staff would need a period of eight months to confidently verify its safety.

The modern centre has 20 beds, 16 day places for chemotherapy patients and seven hostel beds for patients travelling long distances.

The machine's manufacturer has now repaired the equipment and a warning has been sent to other hospitals to check their own machines.

The same radiotherapy equipment is used by Singleton Hospital in Swansea and Velindre in Cardiff, where staff are carrying out checks.

A helpline has been set up on 01745 445 143 for concerned patients. Welsh health minister Jane Hutt said anyone needing reassurance should get in touch.