The first particle was found near Dounreay in 1984

The UK Atomic Energy Authority (UKAEA) said more particles were being found because of the increased sensitivity of detection equipment.

But anti-nuclear campaigners argued that this proved the level of contamination was higher than that claimed by the authorities.

The first so-called hotspot was described as a one-off when it was discovered in 1984.

Washed ashore

However, they have been cropping up regularly on Sandside Beach over the last two years.

The latest finds, which were made on Wednesday and Thursday, bring the total to 25.

The UKAEA denies that more pollution is now being washed ashore.

Dounreay spokesman Colin Punler said more particles were being found because they were using more sophisticated detectors.

The increased incidence of new particles is becoming very worrying Bruce Crawford MSP

However, Lorraine Mann of Scotland Against Nuclear Dumping said this vindicated what she and beach owner Geoffrey Minter had been saying all along - that monitors were only picking up a tiny fraction of the real contamination.

Mr Minter took the UKAEA to the Court of Session earlier this month in a bid to improve Dounreay's monitoring regime.

The Scottish Executive agreed that improved monitoring was behind the most recent discoveries.

However, a spokesman said it was inappropriate to comment on calls for changes to the monitoring process until the court's decision was made known later this year.

The regulations under which the UKAEA monitors, detects and removes particles are laid down by the Scottish Environment Protection Agency (Sepa).

Monitoring process

A spokesman for the agency said: "Sepa's view is that the requirements placed on the UKAEA are sufficient to detect and remove those particles at local public beaches that may present a significant risk to public health."

Scottish National Party environment spokesman Bruce Crawford said: "It is time that we moved beyond a simple monitoring process of the area and a full detailed survey was carried out by Sepa.

"The increased incidence of new particles is becoming very worrying.

"Ross Finnie (environment minister) must order Sepa to undertake this work immediately."