John Major has launched a scathing attack on Boris Johnson’s handling of Brexit, saying his administration agreed to the Northern Ireland protocol despite knowing it was unworkable.

“That must be the first agreement in history that was signed by people who decided it was useless in the first place,” Major told a Westminster committee on Tuesday.

The former Conservative prime minister did not name his successor but expressed astonishment at the acceptance of the protocol, which Johnson used to promote an “oven-ready” Brexit deal in the 2019 election.

He said Britain’s exit from the EU was a “colossal mistake” that had left the UK outside the world’s main three power blocs. “There is America, there is China and there is the European Union. We should be in Europe.”

The blunder was worsened by the agreement to impose checks on goods entering Northern Ireland from Great Britain, he said.

“The protocol is a mess. It was very poorly negotiated,” Major told the Northern Ireland committee. “I think some of the promises made after the protocol that there would be no checks on trade from Britain and Northern Ireland, how those promises came to be made I cannot imagine because they were patently wrong. The protocol needs changing. I am baffled as to how we could have reached a situation where that protocol was accepted.”

Major, who was prime minister from 1990 to 1997, criticised Johnson’s administration for signing the protocol with the EU apparently on the basis that it would be later reformed. He also criticised Johnson and his successor, Liz Truss, for their threats to override the Brexit agreement.

“Even if the protocol bill was wrong that does seem to be a strange way to proceed because that sort of behaviour is pretty unwise. We, the British, would not respond to threats of that sort. Why do we think that the European Union would?”

Major was appearing as a witness at a committee hearing on the effectiveness of the institutions of the 1998 Good Friday agreement. The Democratic Unionist party (DUP) has collapsed power-sharing in Northern Ireland in a protest against the protocol, leaving the Stormont executive and assembly mothballed. The party says the protocol damages the region’s economy and its place in the UK.
Major said all sides: London, Dublin, Brussels and parties in Northern Ireland, would have to compromise. “A statesmanlike response would be to recognise that nobody is going to get everything they wish, but to accept compromise in the interest of returning democratic government to Northern Ireland. That will not be easy for anyone.”

He said it seemed Rishi Sunak’s Downing Street team was making progress in talks with the EU. In a tacit rebuke to the DUP – and possibly also an appeal to Sunak to face down Tory Brexit hardliners – Major counselled compromise. “Statesmen who do that will succeed. Politicians who keep shouting slogans to their most extreme supporters will not.”

Major lauded the contribution of his Irish counterparts, Albert Reynolds and John Bruton, as well as Northern Ireland party leaders, clerics, civil servants, Bill Clinton and Tony Blair in paving the Good Friday agreement. “This is a settlement that has many parents. No one can claim full paternity.”

He expressed concern that poverty in Northern Ireland was undermining peace and reconciliation. “Economic hardship is a divisive force.”