British MPs have approved a controversial bill, which plans to override parts of its Brexit withdrawal agreement with the European Union.
The UK government has admitted that the Internal Market Bill breaks international law, with the EU Commission threatening legal action over what it considers a violation of an international treaty.
It was passed by the House of Commons in a vote on Tuesday night, with a majority of 84.
It means the proposed legislation will move to the British House of Lords.
British Prime Minister Boris Johnson has ploughed ahead with the bill, parts of which will give ministers the power to override the Brexit divorce deal, despite the EU's call for him to withdraw those measures from the proposed legislation by the end of the month.
Brussels has warned it will "not be shy" in taking legal action if the UK government does not do so.
The British government introduced the bill earlier this month which saw anger from both European capitals and at Westminster - including among a sizeable group of its own Conservative MPs.
Many rebel members - including former prime minister Theresa May - abstained on Tuesday.
It has been described as "a threat to the Good Friday Agreement".
Former UK Chief Negotiator for Northern Ireland, and Tony Blair's former chief of staff, Jonathan Powell said: "It is a threat to the Good Friday Agreement because it reopens the agreement reached with the EU, the Northern Ireland protocol, which found a way of balancing the problem and keeping the border between Northern Ireland and the Republic open.
"Basically there are only two choices: if the UK is going to leave the single market and is going to leave the regulatory processes of the EU, you would have to have a border between Ireland and Northern Ireland because those regulatory rules would have to be enforced.
"Essentially what Boris Johnson did is accept [the border] being in the Irish sea, which I thought wasn't ideal but better than the alternative of having it in Ireland."
While Foreign Affairs Minister Simon Coveney has accused Mr Johnson of "inflammatory language" and "spin" over claims the EU is attempting to "blockade" goods travelling across the Irish Sea.
Speaking earlier this month, he said: "There is no blockade proposed and that is the type of inflammatory language coming from Number 10 which is spin and not the truth.
"What has been agreed is that there will be limited checks on goods coming from Great Britain to Northern Ireland because there is an agreement to prevent the need for physical border infrastructure on the island of Ireland.
"Therefore we have to ensure that goods are not travelling from Great Britain to the single market."
US presidential candidate Joe Biden also issued a warning over the plan.
While US House Speaker Nancy Pelosi has warned that any attempt to undermine the Good Friday Agreement will see "absolutely no chance" of a US-UK trade agreement.
The Good Friday Agreement is the bedrock of peace in Northern Ireland. If the U.K. violates its international agreements & Brexit undermines the Good Friday accord, there will be absolutely no chance of a U.S.-U.K. trade agreement passing the Congress. https://t.co/n7E4GHTJcI
— Nancy Pelosi (@SpeakerPelosi) September 9, 2020
Additional reporting: IRN