The Conservative and Labour grandees demanded the Prime Minister drops the UK Internal Market Bill and urged MPs to block it. Mr Blair and Sir John claimed the new Brexit legislation threatens the Irish peace process, will make trade negotiations harder and damages Britain’s integrity.
But the intervention by the two former prime ministers prompted a furious reaction.
Former Conservative MEP Daniel Hannan tweeted: “No doubt John Major and Tony Blair see themselves as distinguished elder statesmen, and perhaps a few others share that view.
“But I’m pretty sure that most voters will see two politicians who spent their careers backing Brussels whether it was right or wrong doing so once again.”
Former Brexit Party MEP Rupert Lowe said: “Tony Blair and John Major working together, what a team that is.
“Blair is an insufferable man desperate to be relevant.
“There’s a reason he’s so wildly unpopular – stay out of it Tony!”
Another Twitter user wrote: “So when it comes to shameful these 2 have some form don’t they?”
One fumed: “If Blair and Major want the Withdrawal Agreement kept it must be very bad news for the UK. Two of the worst PMs we have ever had.”
READ MORE: Brexit activist claims EU has become a threat to peace
But others backed Mr Blair and Sir John’s meddling, with one Twitter user saying: “Wholeheartedly agree with them.”
The former Conservative and Labour leaders condemned Mr Johnson’s new Brexit legislation in an article for the Sunday Times.
Sir John and Mr Blair said: “It puts the Good Friday Agreement at risk, because it negates the predictability, political stability and legal clarity that are integral to the delicate balance between the north and south of Ireland that is at the core of the peace process.
“This has wide-ranging ramifications. It will not only make negotiation with the EU more difficult, but also any trade negotiations with other nations, including the United States. Once trust is undermined, distrust becomes prevalent.
“We both opposed Brexit. We both accept it is now happening.
“But this way of negotiating, with reason cast aside in pursuit of ideology and cavalier bombast posing as serious diplomacy, is irresponsible, wrong in principle and dangerous in practice.
“It raises questions that go far beyond the impact on Ireland, the peace process and negotiations for a trade deal – crucial though they are.
“It questions the very integrity of our nation.”
Mr Johnson’s Bill has proved controversial as it could break international law by flouting the Withdrawal Agreement he sealed with Brussels last October.
But the Prime Minister has insisted that without it, the EU could “carve up our country” and “seriously endanger peace and stability” in Northern Ireland.
Mr Johnson is working to drum up support among Tory MPs for his Bill.
Former prime ministers Theresa May and Gordon Brown have also hit out at the plan.