![[ image: width=150]](/olmedia/445000/images/_446072_mitrokhin2150.jpg)
The claims were made in revelations by Russian dissident Vasili Mitrokhin who handed over the details to MI5 in 1992.
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.
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.
"I do not know what extent these plans progressed beyond discussions - it is in the nature of the secret service that it is very secret."
Government briefed on spy secrets
(13 Sep 99 | UK Politics)
MI5
Prince of Wales
Home Office
The BBC is not responsible for the content of external internet sites.
Labour explores Assembly election wounds
Delays hit coach crash compensation
Ex-headmaster guilty of more sex charges
Museum in pay-off probe
Euro row over "Welsh" money rebate
Germans challenge Cardiff for Tyson fight
Ticket chief resigns after World Cup fiasco
Quick-thinking driver foils 'carjacker'
Spring clean for river
Roman remains may halt rugby centre plan
Olympic rider Sir Harry is mourned
The Welsh Week at Westminster
Hanged farmer had been cautioned
Children take over the Assembly
Assembly security review follows pupil's 'threat'
Opposition parties criticise Health Secretary
Welsh airline is launched
Anglesey power plan may bring 1,500 jobs
Former paratroopers recruited drugs courier
Star's cyber-fans help Welsh charity
Labour election inquest blames party rifts
New factory creates 500 jobs
Bassey will go world wide on the web
French boycott gathers pace
Villagers fume at delay over sewage problem
Pensioner leaves fortune to birds
MP demands return of Glyndwr letter