A UK government bill proposing an overhaul to a post-Brexit deal in Northern Ireland returns to parliament on Monday, despite EU warnings it is illegal and could spark a trade war.

Brussels threatened legal action after the UK government earlier this month introduced the Northern Ireland Protocol Bill to unilaterally change trading terms for the British province.

A day before it gets its second reading in parliament -- the first opportunity for lawmakers to debate a proposal -- the EU's ambassador again warned London of reprisals if it is passed.

"We think it is both illegal and unrealistic. It is illegal because it's a breach of international law, a breach of EU law and UK law," Joao Vale de Almeida told Sky News on Sunday.

"We are committed to find the practical solutions on implementation, but we cannot start talking if the baseline is to say everything we have agreed before is to be put aside," he added.

The protocol -- signed separately from the wider trade and cooperation agreement -- requires checks on goods arriving into Northern Ireland from England, Scotland and Wales, in order to track products that could be potentially headed to the EU via the Republic of Ireland.

This creates a customs border down the Irish Sea, keeping Northern Ireland in the EU's customs orbit so as to avoid a politically sensitive hard border between it and EU member Ireland.

But pro-British parties in Northern Ireland say it is driving a wedge between London and Belfast and are refusing to join in a power-sharing government in the province until the protocol is changed.

EU anger

Unionist parties and the UK government argue the protocol is threatening the 1998 Good Friday Agreement that ended three decades of violence over British rule in Northern Ireland.

They want checks to be removed on goods and animal and plant products travelling from Great Britain.

"The problem (with) the protocol is the way the EU want to see it implemented," the UK's Northern Ireland minister Brandon Lewis told Sky News on Sunday.

"What we are doing is fixing the problems within the protocol, about how it's being implemented, so that businesses can prosper again.

"I want to see the re-establishment of the Northern Ireland Assembly and the executive -- the protocol is getting in the way of that and that's why it's breaching the Belfast Good Friday Agreement," he added.

"We want to do this by agreement with the EU but to do that, they need to show some flexibility."

Plans to unilaterally override parts of the protocol have provoked anger in European capitals, particularly Dublin, and have led to the EU threatening an all-out trade war if implemented.

"Unilateral action is damaging to mutual trust," European Commission Vice President Maros Sefcovic told reporters in Brussels when the UK introduced the legislation.

Sefcovic said Brussels would now consider reopening a suspended "infringement procedure" against Britain, as well as opening fresh cases.

This would be to "protect the EU single market from risks that the violation of the protocol creates for the EU businesses and for the health and safety of EU citizens", he added.

After Monday's debate, the bill still has several hurdles to clear in both the House of Commons and upper House of Lords before it becomes law, and faces legal challenges.