The European Commission has warned it will take "proportionate action" to secure the legal implementation of the Northern Ireland Protocol after the UK government published legislation which will override the mechanism.
The commission's vice president Maros Sefcovic said the EU viewed the UK's actions with "significant concern" and that it would consider what steps to take next.
He said this would begin with the resumption of legal proceedings against the UK, which it suspended in September, over breaching the withdrawal treaty of 2020.
As well as re-starting infringement proceedings against the UK, he said the EU would also look at launching further legal action to protect the integrity of the EU single market, as he ruled out renegotiating the trade protocol.
"Unilateral action is damaging to mutual trust," Mr Sefcovic said.
"Our aim will always be to secure the implementation of the protocol. Our reaction to unilateral action by the UK will reflect that aim and will be proportionate."
Meanwhile, Michelle O'Neill, the vice president of Sinn Fein, said Boris Johnson's attempts to override parts of the protocol are "illegal", adding that the prime minister is "in clear breach of international law".
But the UK government has insisted it is acting in line with the law.
Ministers have said all parties in Northern Ireland recognise there are problems with the protocol, which is designed to prevent a hard border with the Republic of Ireland but effectively creates a border in the Irish Sea with goods imported from Britain subject to customs checks.
The bill will enable ministers to establish a "green lane" so trusted traders are allowed to move goods from Great Britain to Northern Ireland without checks, as long as the products remain within the UK.
Goods supplied by firms outside the trusted trader scheme, or products destined for Ireland and the EU, would go through a red lane and face checks.
A spokesman for Ireland's foreign minister Simon Coveney said the bill "marks a particular low point in the UK's approach to Brexit" and said the plan would "ratchet up" tension and breach the UK's international commitments.
Elsewhere, a majority of Stormont assembly members (MLAs) signed a joint letter to Mr Johnson stating their opposition to proposed legislation to amend the Northern Ireland Protocol.
However, Foreign Secretary Liz Truss said the UK government "is acting within international law" with the new legislation.
"We are changing the protocol, we are not getting rid of the protocol. I think that is important to acknowledge," Ms Truss told broadcasters.
"We have to take the actions to protect the people across the United Kingdom, we have to take the actions to protect peace and stability in Northern Ireland - and that is exactly what we are doing."
The foreign secretary also disputed that the legislation may not be triggered and is being used for negotiating purposes, adding: "We are completely serious about this legislation."
Earlier, the PM insisted the legislation would introduce "relatively simple" changes and said it would be a "gross overreaction" by the EU if they sought to retaliate by triggering a trade war.
The alterations will be debated and voted on by MPs.
Following a conversation between Ms Truss and US Secretary of State Antony Blinken, America's top diplomat said the UK's proposal to override the protocol will not damage the country's current trade dialogues with Washington.
But, he also urged the UK to continue negotiations with the EU, adding: "We recognise there have been challenges over the implementation of the Northern Ireland Protocol.
"We urge the UK and the EU to return to talk to resolve these differences."
The Democratic Unionist Party has refused to support power sharing until the issues are resolved.
Its leader Sir Jeffrey Donaldson, speaking after the bill had been published, said: "The DUP will judge what constitutes decisive action as we see this bill progressing."
The new bill creates a framework to allow ministers at Westminster to introduce changes in four areas covering customs and agri-food safety checks, regulation, subsidy controls and the role of the European Court of Justice.
• provides that certain specified provision of the Northern Ireland Protocol does not have effect in the UK
• gives ministers of the crown powers to provide that other provision of the Northern Ireland Protocol does not have effect in the UK
• provides that enactments, including the Union with Ireland Act 1800 and the Act of Union (Ireland) 1800, are not to be affected by provision of the Northern Ireland Protocol that does not have effect in the UK
• gives ministers of the crown powers to make new law in connection with the Northern Ireland Protocol (including where provision of the protocol does not have effect in the UK
In a legal policy paper, published alongside the bill, the government states the move is justified under international law because of the "genuinely exceptional situation".
"The government recognises that necessity can only exceptionally be invoked to lawfully justify non-performance of international obligations," it reads.
"This is a genuinely exceptional situation and it is only in the challenging, complex and unique circumstances of Northern Ireland that the government has, reluctantly, decided to introduce legislative measures which, on entry into force, envisage the non-performance of certain obligations.
"It is the government's position that in light of the state of necessity, any such non-performance of its obligations contained in the Withdrawal Agreement and/or the protocol as a result of the planned legislative measures would be justified as a matter of international law."
Labour leader Sir Keir Starmer, speaking to broadcasters on Monday, said the government was "going down the wrong track".