Theresa May has attacked the leading Brexit-backing Tory Jacob Rees-Mogg in front of other MPs according to reports, slamming his support for a clean Brexit, outside the Customs Union with the Irish border open.

The Prime Minister is reported to have “confronted” Mr. Rees-Mogg, with The Times describing it as a “clash”, as well as quoting sources who said Mrs. May was “sending a tough signal” to Brexiteer Tories.

All 315 backbencher Tory MPs had been called to a “technical briefing” at Number 10, in which Mrs. May’s chief of staff explained her perception of the pros and cons of the two main customs options.

The issue has sharply divided the Cabinet in recent weeks, with foreign secretary Boris Johnson slamming Mrs. May’s preferred ‘customs partnership’ as “crazy”, and Brexiteers backing the ‘Max Fac’ plan which would allow the UK to control trade policy after the divorce.

During Monday’s session, Mr. Rees-Mogg, who leads the European Research Group, is reported to have suggested to Mrs. May that the border between Northern Ireland and the Republic could be left open after Brexit by using technology.

A source told the newspaper: “Jacob said, ‘If there was a border poll, I have no doubt we would win, as the UK did in Scotland [in the 2014 independence referendum].’

“Mrs May said, ‘I would not be as confident as you. That’s not a risk I’m prepared to take. We cannot be confident on the politics of that situation, on how it plays out.’ ”

Another Tory MP added: “She slapped him down very hard. Everyone thinks he knows what he’s on about but she got him on facts. She was absolutely firm and passionate about the Irish position. I got a sense she realises what really matters.”

Mr. Rees-Mogg did not comment on the exchanges, and with his typical turn of phrase, said the meeting was “courteous and respectful”.

However, he hit back in a column in The Telegraph, appearing to attack Mrs. May’s Brexit negotiating record, implying she was guilty of repeatedly surrendering to their demands.

“If we do not push on with firmness and tenacity the harm being done to our fishermen will continue, our powers to protect our borders will be compromised and our money will be squandered by needlessly paying the EU’s ransom upfront,” he wrote.

He added: “Moreover, it is vital that the Prime Minister, the Secretary of State for DeExEU and Parliament see the process for what it really has become,” claiming, “EU insists upon being intransigent and tries to make an example of the UK” rather than negotiating.