Hardline Tory Brexiteers are expected to back the Government's plan to override post-Brexit trade rules in Northern Ireland.
Legal analysis of the bill by the European Research Group's experts concluded it “properly reinforces” the Good Friday Agreement, which put an end to decades of sectarian violence in the region.
Foreign Secretary Liz Truss will tell MPs on Monday that new legislation will fix the problems that parts of the Northern Ireland Protocol have created and help uphold the peace deal.
But the move has angered other Tories, including former PM Theresa May, who believe it breaks international law and undermines the UK's global reputation.
EU Ambassador to the UK João Vale de Almeida also said the bill, which has its second reading in the Commons, is both “illegal and unrealistic.”
He told Sky News: "I am afraid that the Government’s track is very probably, unfortunately, a road to nowhere and we would like to avoid that…
"We are not dismissing but we read it very carefully and we think, to be very frank, that it is both illegal and unrealistic.
"It is illegal because it is a breach of international law, a breach of EU law, UK law and international law.
"It is a treaty that we signed, ratified and even went through a general election in this country.
"We also believe it is unrealistic because it does not provide a real alternative to the Protocol".
But Ms Truss will tell MPs: "Our overriding priority is protecting the Good Friday Agreement, the bedrock of peace and stability in Northern Ireland - as it stands the Protocol is undermining this delicate balance.
"This legislation will fix the problems the Protocol has created, ensuring that goods can flow freely within the UK, while avoiding a hard border and safeguarding the EU Single Market.
"A negotiated solution has been and remains our preference, but the EU continues to rule out changing the Protocol itself - even though it is patently causing serious problems in Northern Ireland - which therefore means we are obliged to act.”
