MI5 considered Irish playwright Brendan Behan to be “too unstable and too drunken” to be dangerous, previously secret files show.

Behan, best known for his play The Quare Fellow and his book Borstal Boy, spent eight years in prison for IRA activities including the attempted murders of two Dublin detectives.

He was also a supporter of the Communist Party, although his legendary drinking bouts in later life meant that he ceased to be taken seriously as a threat to Britain's national security.

His MI5 file, newly released by the National Archives, includes several curt asides by agents commenting on his dissolute state.

One report, dated June 1956, states: “The attached note on Brendan Behan may possibly be of interest. Source considers that as an individual he is too unstable and too drunken to be particularly dangerous.”

Another document summarises a telephone call between Behan and Communist Party organiser Barbara Niven in August 1957, in which the playwright said he was “where he always was — he was not an English man, he was a communist; the comrades did not always agree with him”.

The MI5 officer noted: “I assumed from the above conversation that Brendan was either a little mad or drunk.”

The file also recounts the time in October 1952 when Dublin-born Behan was arrested at Newhaven, East Sussex, for contravening an expulsion order.

Told by an officer from Scotland Yard's Special Branch he was being detained, he replied: “I will explain everything, but not now as I am suffering from a hangover.”

Behan was fined £15 and placed on a ship for Dieppe in northern France on November 5 1952.

The file includes numerous newspaper cuttings, some reviews of the author's plays and other reports on his drunken antics.

An MI5 memo, dated May 1956, notes that a report about Behan's exploits in the now-defunct Daily Sketch gives a “not very attractive” summary of his personal background, “most of which is confirmed by our records”.

The document continues: “Although nominally a Roman Catholic, he states that his hero is Lenin.”

Another report says a source has learned that Behan has “lost none of his sympathy with the IRA” and is thought to be contributing to its funds.

After battling alcoholism for many years, Behan died in a Dublin hospital in March 1964, aged 41.