Until now there had been nothing more than suspicion that Rees was a Soviet agent.

BBC Wales's Sally Davies: "No evidence that he passed anything of note"

But he has been unmasked in the KGB archives smuggled out of Moscow by Mitrokhin, which also revealed Melita Norwood and John Symonds were spies.

BBC Wales reporter Sally Davies, who has been looking into the claims, said the Mitrokhin Archives revealed that Rees had two code names - Fleet and Gross.

He was regarded as a key element in Guy Burgess's recruitment strategy at Oxford University in the late 1930s.

Goronwy Rees - Guy Burgess wanted him assassinated because of what he might know

"His daughter Jenny Rees said he was just passing on tittle tattle because he was part of the Oxbridge set and knew the famous five who were unmasked," she said.

"She said he was a minor player and not actually a spy because she always maintained that if he were a spy they would have a KGB file on him, well there now appears that there was."

The KGB regarded Rees as a useful contact because of his position as a fellow of All Souls Oxford where he would come into contact with top politicians.

But most commentaries suggest his communist sympathies were ideological rather than political.

Sickened

He was recruited after writing an essay on mass unemployment in the South Wales Valleys.

But the archives confirm that Rees passed no information of any great importance to the Soviet regime - and that he ended his contact after three years because he was so sickened by the Nazi-Soviet pact.