An MI5 file on Mr Donaldson, who led the SNP from 1961 to 1969, claims that he conspired to set up a Vichy-style regime with himself as a "Scottish Quisling" in the wake of Hitler's widely-anticipated invasion.
In 1941, Mr Donaldson revealed to a close confidant - who was also a British agent - that a network of Nazi sympathisers were planning to undermine the war effort.
His United Scotland Movement was said to be contemplating "spreading confusion by false reports and minor acts of sabotage, and is in fact now endeavouring to start a whispering campaign to spread rumours, particularly of shipping losses".
Mr Donaldson told the agent that he believed the German invasion would be a success.
"The government would leave the country and England's position would be absolutely hopeless, as poverty and famine would be their only reward for declaring war on Germany. Scotland, on the other hand, has great possibilities," Mr Donaldson is recorded as saying.
"The movement in Scotland must then be able to show the German government that it is organised and has a clear-cut policy, that it is not with England in the war.
"The German government will give them every possible assistance in their early struggle, and when fire and confusion is at its height in England the movement can start in earnest."
Senior MI5 officer Lieutenant Colonel Dick Brooman-White believed that Mr Donaldson was planning to follow the lead of Vidkun Quisling, who led a puppet government in Norway.
In 1941 Mr Donaldson was among a number of Nationalist figures to be arrested under suspicion of subversive activities.
His internment was widely believed to be a result of his pacifist policies, and he was released without charge after six weeks in Barlinnie prison.
An SNP spokesman hit back, saying: "This is stuff and nonsense and wartime tittle-tattle, which stands as a curiosity of the wartime paranoia of MI5 and nothing more.
"This fanciful account appears to be from the same source as one which claimed that Hugh MacDiarmid, one of Scotland's greatest writers, was a threat to national security while he wrote poetry from his home in the Shetland Isles.
"The fact that even Tom Johnston, the secretary of state at the time, dismissed the story and released Mr Donaldson from internment speaks volumes for the veracity of this far-fetched tale," he said.