“We are very clear that that is legal under international law, and we will publish a legal statement very shortly outlining the government’s legal position,” Ms Truss told BBC Radio’s 4’s Today programme.
Attorney General Suella Braverman has reportedly advised the government that the radical legislation move would be legally sound because of the “unreasonable” way that protocol checks have been implemented.
But MPs and legal experts have questioned Ms Truss’s assertion in the Commons that a proposed bill to streamline checks is “legal in international law”.
“MPs will want to see the legal advice and know it is sustainable and arguable before going ahead with this,” one former Tory minister told The Independent.
“We don’t want to be back in the same battle as we had with the Internal Market Bill, with ministers talking about breaking the law in a limited way.”
Catherine Barnard, a professor of EU law at Cambridge University, said there was no legal basis to the government’s argument that the Good Friday Agreement takes precedence over protocol provisions.
“The mainstream view is that this is likely to be incompatible with international law,” she said. “It will certainly be incompatible with the UK’s withdrawal agreement.”
Ms Truss told the Commons on Tuesday she intends to bring forward the bill – aimed at creating a check-free “green lane” for goods coming from GB to NI – “within weeks”.
The foreign secretary defended the controversial plans again on Wednesday, insisting the action to address the “very severe” situation in the region cannot be delayed.
She told Times Radio: “We can’t delay delivering a solution in Northern Ireland. We haven’t seen the [Northern Ireland] Executive form since February. So we do need to make these changes. And these changes will … make it better for everyone.”
Ireland’s deputy prime minister Leo Varadkar has said the move to disapply parts of the protocol without an agreement with the EU is “not very respectful” to the British people.
Mr Varadkar pointed out that 59 of the Stormont Assembly’s 90 MLAs do not want to ditch the protocol, and the British public voted for the Brexit deal that is currently in operation.
He told RTE: “If they keep trying to impose on Northern Ireland things that Northern Ireland doesn’t want, that drives more people towards nationalism and away from support for the Union … it just seems a bit puzzling.”
When asked about potential EU retaliation and the possibility, Mr Varadkar said the UK would have to “do something” for Brussels to take action.
Mujtaba Rahman, analyst at the Eurasia Group consultancy firm, said the EU would “not overreact at this stage” since Brussels chiefs are aware it could take six to 12 months for the legislation to move through parliament.
The expert told The Independent that he expected the European Commission to start “preparatory work” on possible retaliatory moves – including tariffs and the suspension of the Brexit Trade and Cooperation Agreement (TCA) – as a bill moves through parliament.
Labour’s chancellor Rachel Reeves said on Wednesday that the EU was being “overzealous” over checks on goods destined to stay within the UK.
She added: “But the way to resolve this is not through megaphone diplomacy, it’s not unilaterally ripping up the protocol, it’s by working in partnership to resolve these very real issues that do exist.”