The claims were made in revelations by Russian dissident Vasili Mitrokhin who handed over the details to MI5 in 1992.

Professor Gareth Jones: "It is conceivable the Russians saw there was opposition to the Investiture"

The plot, codenamed "Operation Edding" targeted a bridge on the route along a road from Porthmadog to the Investiture at Caernarfon Castle.

The plan was to destroy the bridge a month before the event itself and blame the British security services, but it was scrapped at the last minute.

Two Welsh extremists did attempt to sabotage the north Wales coastal rail line in a different part of north Wales shortly before the Investiture, but were killed when their bomb exploded before they reached their objective.

The pair, George Taylor and Alwyn Jones, became known in nationalist circles as the "Abergele Martyrs." Their families said the men had not intended to kill Prince Charles and were nothing to do with the KGB.

The plot is one of several extraordinary claims in the so-called "Mitrokhin archive", which includes details of plots against the Pope, against Rudolph Nureyev and a campaign to blame the CIA for President Kennedy's assassination.

Russian dissident Vasili Mitrokhin who handed over details to MI5 in 1992

The revelations coincide with the disclosures that great-grandmother Melita Norwood was a Russian spy in the 1930s and that a former Scotland Yard detective also allegedly worked for the Russians.

'Very plausible'

Former MI5 officer David Shayler has told BBC News Online that he knows the names of at least two more spies who have not been prosecuted.

Professor Gareth Jones, a professor in Russian history at Bangor University, said the claims about the Caernarfon bomb plot were "very plausible".

"You have to remember that in 1969 we were in the middle of the Cold War," said Prof Jones.

He said it was conceivable the KGB would exploit every incident in the West to create trouble.

'Creating trouble'

"The Russians would have seen there was opposition to the Investiture at the time.

"They would have understood that and they could see there was a possibility of creating trouble in Britain.