NHS founder Aneurin "Nye" Bevan was spied on by MI5 for more than 20 years, according to newly released files.
Papers given to the National Archives show he first came to the attention of the security service in the 1930s when he was considered a firebrand left-winger linked to prominent communists.
Monitoring continued after Bevan, from Tredegar, Blaenau Gwent, became a minister in the post-war Labour government when he drove through the creation of the NHS in the face of opposition from doctors.
At one stage the then MI5 director general became so concerned about his links to a Russian spy that he went to see Prime Minister Clement Attlee in No 10 to warn him.
But some in the service questioned why it was taking an interest in him, given everything he did was in the open.
After his death in 1960, one MI5 officer called the content of the file "remarkably tame".
The papers show MI5 first opened a file on Bevan when he was backbench Labour MP for Ebbw Vale in 1933.
One of the first reports included an account of an anti-Nazi rally in Montreal, Canada, which Bevan addressed in 1934.
"Bevan has inherited the evangelical fervour of his Welsh forebears and knows how to use it in the Communist cause," it said.
In 1943, Roger Hollis, future director general, summarised what they knew of Bevan's activities.
In 1933 he said the MP had been "in close touch" with Communist Party of Great Britain general secretary Harry Pollitt and party members who backed his plan to organise "workers' freedom groups" to defend against "capitalists".
Although there was nothing in the files to indicate MI5 tapped his phone or intercepted his mail, some wondered why they were watching him at all.
"I do not think that Bevan's activities are a matter in which we, the security service, should be involved," one officer wrote in 1943.
"In fact he makes exactly the same remarks in the Houses of Parliament pretty frequently."
The reporting continued after the war when Bevan was made health minister in Attlee's government.
In 1949 MI5 became increasingly concerned about Bevan's friendship with Wilfred Macartney, "an unsavoury, unscrupulous character" convicted of spying for Russia in 1928.
Recordings of conversations suggested he was passing information obtained by Bevan to Communist Party leader Pollitt.
That May MI5 chief Sir Percy Sillitoe took evidence to Attlee, who dismissed Macartney's claims as a "tissue of lies" but agreed to warn Bevan about him.
One of the final reports dates from 1954 when Bevan, now back in opposition, was invited to meet the Soviet ambassador, Yakov Malik, at London's Polish embassy.
The event "went off very badly", partly because the Russian had difficulty in understanding Bevan's accent, which led to misunderstanding.
"The party broke up in a none too friendly atmosphere," the report said.
The file was not closed until 1975 - 15 years after Bevan died.