The crossbench peer, who retired from the House of Lords this summer following a Parkinson’s diagnosis in 2019, also cautioned against any proposals giving the cabinet secretary, Dame Antonia Romeo, legal powers to direct and overrule officials who run government departments.

“I’m a friend and admirer of the current cabinet secretary, so I don’t want to be unkind to her. But if I was a permanent secretary, I wouldn’t put up with it, nor would my Secretary of State,” he says.

In a wide-ranging conversation with the Lord Speaker, Lord Forsyth of Drumlean, on the Lord Speaker’s Corner podcast, Lord Hennessy stresses the importance of the prime minister being the “number one guarantor of constitutional decency”, responsible for observing rules including the Ministerial Code and Cabinet Manual.

He says one of his “greatest shocks” in recent years was seeing “just how vulnerable we were when we had a prime minister in Number 10 who either didn’t know about the niceties of the British constitution, the careful balances, or couldn’t give a fig for them, and disdained what we were there for”.

“The British constitution belongs to all of us,” he says.

He also warns that a future government with a big majority could “rip up the constitution and the way we recruit and promote our senior civil servants and interfere with the choice of judges”.

“It would be very easy if they had a big enough majority for them to take away the fundamentals which have served us so well … so much would go. We’d crumble and it would be very hard to get it back,” he added.

Discussing his Parkinson’s, he says the condition comes in “shock waves” but he has been able to create his own “personal Workers’ Education Association”, an informal adult education group composed of friends, who, with his wife Lady (Enid) Hennessy, have helped him adjust, “which is not easy, from a pretty fast-paced existence”. Enid also talks on the podcast.

Elsewhere on the podcast, he commented: “Packing the House of Lords with political appointees isn’t necessary, citing Clement Attlee’s post-war government fulfilling its manifesto commitments despite having a small minority of Labour peers.”

He recalled how the late Queen once told him: “The British constitution has always been puzzling and always will be.’ And I thought but didn’t say, ‘But you’re it, dear.’”