Jacob Rees-Mogg has criticised Theresa May over her handling of Brexit negotiations, calling the Prime Minister's stance on the Irish border "a mistake".
The arch-Brexiteer said Ms May was wrong to refuse to play hardball with Brussels by daring them to set up a hard border in Northern Ireland if there is no Brexit withdrawal agreement.
He also criticised the Cabinet's decision to approve a "backstop" option that would see Northern Ireland fall under the same regulations as the rest of Europe for a time-limited period if no other deal is reached, describing it as a "real problem".
Mr Rees-Mogg has previously urged Ms May to "call their bluff" during talks and argue there will only be a hard border after Brexit if the EU creates one, thereby giving Brussels sole responsibility for finding a solution.
He said: "Ireland has said it doesn't want a hard border, the UK has said it doesn't want a hard border and the EU itself has said it doesn't want a hard border - so frankly it's up to Brussels if it wants to start putting up border posts. We should call their bluff."
He was reportedly rebuked by Ms May for the suggestion.
He told the BBC's Andrew Marr Show yesterday that the Prime Minister's refusal to adopt his proposed approach was "an error". He said: "The Prime Minister said in her Mansion House speech that she wasn't going to do this. I think that is a mistake. I think it is the obvious negotiating position for us to have.
"Bearing in mind the Irish economy is heavily dependent on its trade with the United Kingdom, it is overwhelmingly in the interests of the Republic of Ireland to maintain an open border with the United Kingdom.
"I think if you're going into a negotiation you should use your strongest cards, and just to tear one of them up and set hares running on other issues is, I think, an error."
Mr Rees-Mogg also criticised the Cabinet's decision to agree a "backstop" option that would see Northern Ireland maintain the same regulatory system as the rest of the EU customs union for a "time limited" period if no deal is reached to avoid a hard border.
He said: "That's a real problem because the customs union on its own does not solve the seamless border issues… you require regulatory alignment as well, and that means the single market.
"If we were to stay as a rule-taker, as a vassal state, for an indeterminate period, I don't think that would be delivering on Brexit. And if you offer a backstop that is more attractive than anything you're likely to negotiate, from the other side's point of view, the backstop ends up becoming a front-stop."
He added: "There are concerns, inevitably, at the way negotiations are proceeding."