David Frost's admirers in the Conservative party have welcomed the European Commission's proposals on the Northern Ireland protocol as evidence that his tough negotiating style has worked. Writing in the Daily Telegraph, former party leader Iain Duncan Smith said Frost had shown that the EU understands the language of determined strength.

"He has learned vital lessons from our experience in previous negotiations, when EU officials dug their heels in and laughed up their sleeves as we signed up to their terms. The most important element of the EU's response to British proposals yesterday was not the detail, but the fact that we are now in a renegotiation - something which Frost's critics said would never happen," he said.

But Duncan Smith, who along with every other Conservative MP trooped through the lobbies at Westminster to vote for the protocol less than two years ago, warned Frost against trying to negotiate a series of easements and derogations.

"The EU wants nothing more than to hang on to the notion that the protocol, as originally agreed, remains essentially intact. If he goes down that road it will leave us tied up in complicated EU law and ultimately ruled on by the European Court of Justice (ECJ), no matter the shape of an arbitration panel placed under it," he said.

By offering such sweeping easements to how the protocol is operated, the commission has exposed itself to the charge that, if such changes are possible within the agreement it should have offered them before now. It also runs the risk that an emboldened Frost will simply pocket the concessions and try to wring out more, before collapsing the negotiations over the issue of the ECJ's role in adjudicating how EU single market rules are enforced in Northern Ireland.

In his speech in Lisbon on Tuesday, Frost complained about the EU's frequent accusations that Britain, which introduced legislation last year to overturn the protocol, was a bad faith negotiator. A few hours later, Boris Johnson's former chief adviser, Dominic Cummings, tweeted that the British government had never intended to implement the protocol.

Cummings said they agreed to the protocol in October 2019 in order to get a Brexit deal done but had always planned to "ditch the bits we didn't like" after winning a general election two months later. Cummings absolved Johnson of lying on the basis that Johnson didn't have "a scooby doo" what he was signing up to. But the DUP's Ian Paisley said on Wednesday that Johnson had promised him in October 2019 that he would tear up the protocol after signing it.

"Boris Johnson did tell me personally that he would, after agreeing to the protocol, he would sign up to changing that protocol and indeed tearing it up, that this was just for the semantics," he told the BBC.