The playwright Alan Bennett has said that he is wholly on the side of Edward Snowden, the whistleblower behind revelations of widespread US surveillance on phone and internet communications.

In an interview with BBC Four, Bennett also defended the motives of the Cambridge spies, British agents who worked for the Soviet Union during the cold war and who were depicted in the writer's 1988 play, Single Spies.

"Spying is excusable because they thought that they were doing something to improve things, that they were morally on the right side," he said in extracts published in the Radio Times ahead of the broadcast of an interview in which he also discussed his cancer scare, the Queen, his parents, reaching 80 and how he is "ill-read".

"None of the spies spied for money," Bennett said. "They all did it out of conviction, it was not to do with material gain. The treason they're supposed to have committed doesn't nowadays seem to me to be a particularly important crime. The Edward Snowden stuff – I'm wholly on his side."

Bennett also suggested that he was much more impressed with contemporary US literature than its English counterpart. "I like Philip Roth, for instance. I don't feel any of the people writing in England can tell me very much," he said.

Bennett, who turns 80 on Friday, said writing had become tougher with age and that the output he had already produced did not give him any comfort.

Looking back on his life, he said the later part of it had been much happier than the earlier part – crediting meeting his partner, the journalist Rupert Thomas, with whom he is in a civil partnership.

• This article was amended on 6 May 2014. It originally stated that Bennett's interview was with BBC Radio 4. It was with BBC Four. This has been corrected.