A Cabinet minister said he expects the UK would “break international law” with its proposals to change how a key agreement with the EU operates.

Northern Ireland Secretary Brandon Lewis told MPs it would be in a “very specific and limited way”, adding there are “clear precedents” for the UK and other countries which need to consider their international obligations as circumstances change.

His Labour counterpart Louise Haigh described the admission as “absolutely astonishing” and warned it would “seriously undermine” the UK’s authority on the international stage.

I would say that yes this breaks international law in a very specific and limited way'

The Government will introduce the Internal Market Bill on Wednesday, which aims to ensure goods from Northern Ireland continue to have unfettered access to the UK market while making clear EU state aid rules – which will continue to apply in Northern Ireland – will not apply in the rest of the UK.

Concerns have been raised that key parts of the Withdrawal Agreement, which sealed the UK’s departure from the EU in January, will be overridden by the legislation.

Mr Lewis told MPs the Government is “fully committed” to implementing the Withdrawal Agreement and the Northern Ireland protocol.

I’m not sure you can be a little bit illegal. It’s a bit like being a little bit pregnant.

But he added the UK is taking “limited and reasonable steps to create a safety net” to allow it to deliver on its commitments to the people of Northern Ireland and keep in line with the protocol should outstanding issues not be resolved in talks with the EU.

Mr Lewis told MPs: “The UK internal market legislation that we will bring forward this week delivers on our commitment to legislate for unfettered access, something Northern Ireland businesses have consistently asked us to do and to ensure that we deliver certainty.

“This will give the certainty that the people and businesses, the economy of Northern Ireland, has been asking for and it supports the delivery of the protocol in all circumstances in line with the approach we set out in our command paper in May.

“The safety net we will implement, and we will outline this week, will deliver on the commitments made also in (our) general election manifesto.

“Specifically we will implement the provision in the protocol that Northern Ireland is fully part of the UK customs territory by ensuring that goods moving within the UK will never even inadvertently have to pay EU tariffs.

“We will ensure that businesses based in Northern Ireland have true unfettered access to the rest of the United Kingdom without paperwork and we will ensure that there is no confusion about the fact that while Northern Ireland will remain subject to the EU state aid regime for the duration of the protocol, Great Britain will not be subject to EU rules in this area.”

Conservative Sir Bob Neill, chairman of the Justice Select Committee, later asked: “The Secretary of State has said that he is committed and the Government are committed to the rule of law. Does he recognise that adherence to the rule of law is not negotiable?

“Against that background, will he assure us that nothing that is proposed in this legislation does or potentially might breach international legal obligations or international legal arrangements that we have entered into?”

Mr Lewis replied: “I would say to (Sir Bob) that yes this breaks international law in a very specific and limited way.

“We are taking the power to dis-apply the EU concept of direct effect required by Article 4 in a certain, very tightly-defined circumstances.”

He added that “there are clear precedents for the UK and indeed other countries needing to consider their international obligations as circumstances change”.

Alliance Party leader Naomi Long tweeted: “The Secretary of State for NI has just conceded in Parliament that Govt are about to break international law. His defence seems to be that ‘it’s only in a very limited way’.

“I’m not sure you can be a little bit illegal. It’s a bit like being a little bit pregnant.”

Tory former party leader Iain Duncan Smith said the Government was “quite within its rights” to revisit the Withdrawal Agreement.

Conservative former minister Sir John Redwood said the EU must abide by its commitment to respect the restoration of UK sovereignty and work for a free trade tariff-free agreement, telling the Commons: “If the EU kept its word on these two colossally important points, the problems they’ve created in Northern Ireland would disappear.”