Michel Barnier, the EU’s chief Brexit negotiator, said Mr Johnson’s plan represented “real progress” but that they breached “red lines” for Brussels and insisted the UK would have to make even more major concessions.

But it was Mr Varadkar’s comments that caused the most concern in Downing Street, as they appeared to indicate that he believes Mr Johnson is taking the British people out of the EU against their will.

The Irish Taoiseach said he did not understand how the UK envisages Northern Ireland and Ireland operating under different customs regimes without the need for checkpoints, as Mr Johnson proposes, then went on to suggest that Britain does not want to leave the EU at all.

He said there were several ways of avoiding a hard border in Ireland, including Britain remaining in the EU, adding: “All the polls since Prime Minister Johnson became prime minister suggest that's what the British people actually want, but their political system isn't able to give them that choice."

Whitehall sources diplomatically described his comments as “unhelpful” and Number 10 responded by reiterating that: "The UK voted to leave the EU and the Prime Minister believes it is vital we deliver upon that decision.”

The DUP’s deputy leader, Nigel Dodds, said Mr Varadkar’s “incendiary and outrageous comments” were “a clear ramping up of rhetoric designed to derail any realistic prospect of a deal”.

He added: “The flippant Dublin reaction to the Prime Minister’s proposals has also exposed the reality that the Irish government would never have consented to the United Kingdom leaving the backstop if it had been implemented.

“Our message to Leo is simple. He should reflect on his comments and his intransigent approach. He is destined to go down in history as the Taoiseach who restored a hard border between Northern Ireland and the Republic of Ireland because his friends in Brussels will insist on it.”

Nigel Evans, the Brexiteer Tory MP, said Mr Varadkar was “trespassing on another country’s sovereignty” and “should worry more about what the people of Ireland are thinking”, while David Jones, the former Brexit minister, said Mr Varadkar was “deluded” and would be “well advised” to engage with Mr Johnson’s plan because Ireland has more to lose from a no deal Brexit than any other EU member state.

Mr Varadkar’s comments reflect a growing belief among some EU leaders that Britain could yet revoke Article 50 if Parliament forces a Brexit extension and Jeremy Corbyn - who has promised a second referendum - gets into power.