Nigel Farage's Brexit party has vowed to fight "tooth and nail" against any deal that Boris Johnson negotiates with the EU that largely retains the Withdrawal Agreement.
The prime minister proposed instead that the UK government simply give firm commitments to resolving concerns about the future relationship between Northern Ireland and the Irish Republic.
"This government will not put in place infrastructure, checks, or controls at the border between Northern Ireland and Ireland," Johnson wrote. "We would be happy to accept a legally binding commitment to this effect and hope that the EU would do likewise."
Senior Brexit party officials reacted with fury to the letter, which appeared to suggest that the rest of the agreement – which was negotiated by Johnson's predecessor, Theresa May – was acceptable.
Farage tweeted on Monday night that "even without the backstop, this is still the worst 'deal' in history".
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Lance Forman MEP agreed, saying: “Removing the backstop is not enough. [The Withdrawal Agreement] is a terrible agreement drawn up by Remainers. We will be stuck with it forever. Not a great legacy. All we need is a Free Trade Deal. That works for both parties. Win-win.”
And party chairman Richard Tice was equally forthright, tweeting: “Let’s be clear: the WA even without the backstop is still the worst deal in history, is not Brexit and the Brexit Party will fight it tooth and nail.
“This letter confirms our fears that Boris and the Tories cannot be trusted to deliver a proper Brexit.”
The party also released a new digital ad campaign that featured a Post-it note of what the “Tory Withdrawal Agreement means”. “Northern Ireland Backstop” is crossed out but eight other items remain, including “We pay the EU £39bn”, “British fishing subject to quotas” and “EU sets British tax rates”.
“We need a Clean-Break Brexit by October 31st,” the advert proclaims.
The row appears to mark the first efforts of the party to put clear blue water between it and the Tories on Brexit by maintaining a stance of “No deal is the best deal”.
Since it became apparent Johnson was going to be the next prime minister, the Brexit party has slumped from first place in the polls at 26% shortly after the European elections to fourth and 14% as the new Tory leader has stripped it of Leave-backing voters.