The number of radioactive particles from Dounreay threatening to contaminate a public beach two miles from the plant is six times greater than previously thought, according to experts.

The Dounreay Particles Advisory Group, which advises the Scottish Environment Protection Agency (Sepa) on the hundreds of thousands of particles believed to have been dispersed from the Caithness plant in the early-1960s, said that around 400 or 500 particles were extending into Sandside Bay on the north coast.

The group, issuing its fourth report, graded the particles according to risk and estimates the chances of a significant, or potentially dangerous, particle appearing on Sandside as one in 1000.

It added that, with the recommended fortnightly monitoring of the beach taking place, the chances of anyone coming into contact with a significant particle are one in 60 million.

But Geoffrey Minter, the owner of Sandside Estate, said that monitoring should be taking place daily. He said: "If this was Brighton beach the authorities wouldn't be letting anybody on to it at all because they can't say that significant particles won't come ashore.

"The fact they have underestimated the flow of particles to Sandside comes as no surprise. It is what we have been saying for years. But they are still talking about fortnightly monitoring. What we need is daily monitoring."

Up to £25m is being spent on retrieving the most dangerous radioactive particles that escaped from Dounreay 40 years ago.

Only around 1500 particles, most of which are the size of a grain of sand, have been found since monitoring began in 1983, although tens of thousands more are thought to have escaped.

In February last year the UK Atomic Energy Authority was fined more than £100,000 for the unauthorised release of the particles. Most have been found on the foreshore and seabed below Dounreay.

In 1984 a particle was found on Sandside beach, two miles to the west of Dounreay. Since then a further 108 have been found at Sandside, and it had been estimated that dozens more might further threaten the bay.

The Dounreay Particles Advisory Group's report, which was published yesterday, concluded: "The present estimates of the total number of particles in the main plume offshore from Dounreay are very similar to those in our third report, being only 4% to 8% greater.

"However, the total number of relevant and minor particles in the western plume extending into Sandside Bay is now estimated as about 400-500, which is about six times greater than estimated previously."

Simon Middlemas, director of Dounreay Site Restoration Ltd, the UK Atomic Energy Authority's successor now charged with the decommissioning of Dounreay, said: "The expert work carried out by the Dounreay Particles Advisory Group over several years has made an enormous contribution to our understanding of this problem.

"Without their expert scrutiny advice we would not have been able to begin cleaning up the seabed and so begin recovering the remnants of past practices."