The Ministry of Defence (MoD) has again missed hundreds of radioactive particles contaminating the beach at Dalgety Bay on the Firth of Forth, prompting accusations that it cannot be trusted to carry on monitoring.

In the latest surveys of the Fife foreshore last month the MoD found and removed 83 radioactive particles. A survey of half the same area shortly afterwards by the Scottish Environment Protection Agency (Sepa) found and removed a further 228 particles.

The failure of the MoD to detect most of the contamination was described a "cause for concern" by Sepa. "This reinforces the need to have credible investigation plans in place," said a Sepa spokeswoman.

In December, the Sunday Herald reported that previous MoD monitoring of Dalgety Bay had missed 442 radioactive hotspots. If the MoD fails to come up with clean-up plans by the end of this month, Sepa has said it will formally designate the bay as Britain's first radioactively contaminated land.

The latest news was condemned as "absolutely disgusting" by Annabelle Ewing, the SNP MSP for Mid Scotland and Fife. "All faith, if there was any left, has now been totally lost in the MoD – we can't trust it to monitor the site," she said.

She said the MoD had now missed at least 670 radioactive particles, adding: "It's time for urgent action."

Radiation expert Fred Dawson, who worked for the MoD for 31 years until 2009, accused his former employers of failing to take the pollution seriously. 'The time is well past for further MoD surveys," he said.

He added: "The MoD in all probability is the polluter and should fund all measures Sepa deem necessary to remediate the beach so it is fit for unrestricted use by the public."

The Dalgety Bay Particles Advisory Group, which was set up by Sepa, met on Friday and expressed "continuing concern" at the discovery of more contamination.

Last week, UK Defence Minister Andrew Robathan visited Dalgety Bay and handed over a draft plan of action to Sepa. He declined, however, to accept liability for the pollution. "Who knows who's liable?" he said.

According to Sepa, which is currently considering the MoD's draft plan, the contamination came from the dismantling of planes at a former military base in Dalgety Bay.

The MoD accepted there were "differences" between its monitoring and Sepa's, a spokesman said.

He added: "MoD has welcomed and adopted the monitoring standard recently agreed by the independent expert group. This will provide greater consistency of monitoring."