The EU has threatened the UK with legal action if it does not ditch a bill to override key parts of the Brexit divorce deal “by the end of the month”.
In a statement following emergency talks between the two sides, the EU said the bill “seriously damaged trust between the EU and the UK”.
The UK government said its Parliament is sovereign and can pass laws which breach the UK’s treaty obligations.
Meanwhile, trade talks between UK and EU officials continue in London.
The source of the EU’s concern is Mr Johnson’s proposed Internal Market Bill, which was published on Wednesday.
It addresses the Northern Ireland Protocol – an element of the withdrawal agreement designed to prevent a hard border returning to the island of Ireland.
The bill proposes no new checks on goods moving from Northern Ireland to Great Britain. It gives UK ministers powers to modify or “disapply” rules relating to the movement of goods that will come into force from 1 January, if the UK and EU are unable to strike a trade deal.
The publication of the bill prompted emergency talks between Cabinet Office Secretary Michael Gove and European Commission Vice-President Maros Sefcovic.
Following the discussions, the EU issued a strongly-worded statement warning that the withdrawal agreement was a legal obligation adding “neither the EU nor the UK can unilaterally change, clarify, amend, interpret, disregard or disapply the agreement”.
The EU rejected the UK’s arguments that the bill is designed to protect peace in Northern Ireland arguing that “it does the opposite”.
EU Commission Vice-President Maroš Šefčovič stated that if the bill were to be adopted, it would constitute an extremely serious violation of the withdrawal agreement and of international law.
He urged the government to withdraw the bill “by the end of the month” and “reminded the UK government that the withdrawal agreement contains a number of mechanisms and legal remedies to address violations of the legal obligations contained in the text – which the European Union will not be shy in using”.
Mr Johnson has defended the bill, saying it would “ensure the integrity of the UK internal market” and hand power to Scotland and Wales, while protecting the Northern Ireland peace process.
But critics – including a growing number of Conservative MPs – say the move will damage the UK’s international reputation after a minister admitted the plans break international law.
Labour leader Sir Keir Starmer urged the government to consider “the reputational risk that it’s taking in the proposed way forward”.