The UK's Foreign Secretary Dominic Raab (right) reets German Foreign Minister Heiko Maas at Chevening House, London on Thursday. The E3 (France, Germany and the UK) foreign ministers met at the Foreign Secretary's country residence Chevening House. Britain previously said it may break international law by rewriting parts of its Brexit divorce treaty. (Photo: AFP)

British Prime Minister Boris Johnson has accused the European Union (EU) of threatening to tear the UK apart by imposing a food "blockade" between Britain and Northern Ireland, throwing new fuel on the fire of simmering Brexit talks.

Writing in Saturday's Daily Telegraph newspaper, Johnson said the EU's stance justified his government's introduction of new legislation to rewrite its Brexit withdrawal treaty - a bill that is causing deep alarm among former prime ministers and his own MPs.

Talks between London and Brussels on a future trading relationship are deadlocked as both sides struggle to prize apart nearly 50 years of economic integration, after British voters opted for a divorce.

"My assessment is that an unregulated situation [no deal] would have very significant consequences for the British economy," German Finance Minister Olaf Scholz warned after an EU meeting in Berlin.

"Europe would be able to deal with it and these would not be particularly difficult consequences after the preparations we have already made," he added.

But absent a deal by the end of 2020, when the full force of Brexit kicks in, Johnson said the EU was bent on an "extreme interpretation" of rules for Northern Ireland under the divorce treaty both sides signed in January.

"We are being told that the EU will not only impose tariffs on goods moving from Great Britain to Northern Ireland, but that they might actually stop the transport of food products from GB to NI," he wrote.

"I have to say that we never seriously believed that the EU would be willing to use a treaty, negotiated in good faith, to blockade one part of the UK, to cut it off, or that they would actually threaten to destroy the economic and territorial integrity of the UK."

Johnson said the EU's stance would "seriously endanger peace and stability in Northern Ireland."

"We disagree," former prime ministers Tony Blair and John Major, who led Britain through historic peace talks in the 1990s, retorted in the Sunday Times.

"The government's action does not protect the [1998] Good Friday Agreement - it imperils it," they wrote in a joint opinion piece, calling Johnson's claims to have only belatedly unearthed problems in the EU treaty "nonsense."

"As the world looks on aghast at the UK - the word of which was once accepted as inviolable - this government's action is shaming itself and embarrassing our nation," the piece said.

The EU has threatened Britain with legal action unless it withdraws its unilateral changes by the end of September, and leaders in the European Parliament on Friday threatened to veto any trade pact if London violated its promises.