Writing in The Times, the former Supreme Court justice says that if Prime Minister Boris Johnson “had been not just bad but mad” he could have wrecked the constitution by asking the Queen for a general election.

He states: “Last Thursday’s events marked the triumph of constitutional conventions. The civil service measured up to its reputation for honesty and impartiality. Parliament showed that even party discipline and a large majority will not protect a PM who defies the standards on which our politics depends. Johnson trampled on the conventions of his office. The political cost was high. He lost his job. That is how conventions are supposed to work.”

What is needed is a new arbiter to safeguard the constitution. It cannot be formed from the courts, Lord Sumption argues, because conventions are not legal rules. While the Queen fulfils the function of arbiter through the Privy Council, those who advise her on the use of critical powers are “serving government ministers”.

“There is no safeguard against ministerial abuse. This should change. The council should have a constitutional committee, chaired by a senior retired judge but otherwise consisting of elder statesmen, which could step in with authoritative constitutional advice independently of the government whenever it was needed,” Lord Sumption writes.