Downing Street’s accusation that the EU threatened to block British food from entering Northern Ireland in the case of a no-deal Brexit is an explosive, high stakes claim.

“I’m afraid it has been said to us explicitly in these talks that if we are not listed [as a trusted third country] we will not be able to move food to Northern Ireland,” tweeted David Frost, the UK’s chief Brexit negotiator, at the weekend. “The EU’s position is that listing is needed for Great Britain only, not Northern Ireland. So if Great Britain were not listed, it would be automatically illegal for Northern Ireland to import food products from Great Britain.”

This is a very serious accusation. If acted upon, such a move by the EU would arguably breach the Good Friday Agreement as it would severely disrupt British sovereignty over Northern Ireland without the consent of the people or government of Northern Ireland. The free transportation of agrifoods is essential to any state; an external power having the power to suspend it would render the UK customs union obsolete. It would also breach the Withdrawal Agreement, which both recognises “that Northern Ireland is part of the customs territory of the United Kingdom” and safeguards against “diversion of trade”.

Frost’s tweet seems to have given more sceptical Conservative MPs a reason to support the Internal Market Bill, which will go through its second reading in the House of Commons this evening. The primary criticism of the Bill, that it threatens to breach obligations made in the Withdrawal Agreement, is severely undermined if the EU suggested it would do precisely the same thing.

But the details of the accusation remain vague. We do not yet know the context in which EU officials made the threat, or who exactly it came from. A one-off riff in a tense meeting can be understood in a different light to a pre-prepared statement to David Frost from EU Chief Negotiator Michel Barnier. It’s also not clear whether the threat was made entirely through the implications of Britain not receiving trusted third country status or whether Northern Ireland was explicitly mentioned. Some on the EU side will argue that trust third country status for sanitary and phytosanitary (SPS) measures was never guaranteed, and that rather than a threat, this has been the default since Britain signed the Withdrawal Agreement in January.

The government’s account certainly hasn’t – so far – convinced former Attorney General Geoffrey Cox, who came out against the Internal Market Bill today. Cox’s intervention will be particularly painful because he has built up credibility on both sides of the Brexit divide by positioning himself as an honest broker. His honesty about the lack of a unilateral exit mechanism from the Northern Ireland Protocol while in Theresa May’s cabinet provided Brexiteer rebels with justification to vote against her bill, and Cox was both a Brexit supporter in the referendum and an ally of Boris Johnson in the Conservative leadership election.

Today, he wrote for The Times: “I am a strong supporter of this government and of Brexit and I am deeply saddened to have to say this. We, the British government and parliament, have given our word. Our honour, our credibility, our self-respect and our future influence in the world all rest upon keeping that word. Nothing less is worthy of Britain.”

Former Prime Minister David Cameron also intervened, saying: “Passing an act of parliament and then going on to break an international treaty obligation is the very last thing you should contemplate. It should be an absolute final resort, so I do have misgivings about what’s being proposed.” He becomes the fifth former prime minister to criticise the bill, alongside Theresa May, Gordon Brown, Tony Blair and Sir John Major.

But both Cameron and Cox hedged their denunciations. Cox said he would only “withhold” his support for the bill, suggesting he will abstain rather than vote against, and Cameron said he would refrain from speaking out more vehemently because “the bigger picture is we are in a vital negotiation with the European Union to get a deal, and I think we have to keep that context, that big prize in mind”.

The Internal Market Bill was easily passed its second reading this evening, with only a small rebellion from hardcore critics. Boris Johnson’s government won the vote in the House of Commons with a majority of 77 with 340 votes for the Bill and 263 against. MPs also divided by a margin of 213 to 349 against a Labour amendment designed to cancel the Bill – on this, the government’s majority increased to 136 as the DUP opposed the amendment and the SNP abstained.

The real test for the government will come on Wednesday, when MPs vote on an amendment by Sir Bob Neill which would establish a parliamentary lock on the government’s ability to use the powers it grants itself in the bill.

Throughout this week, Downing Street will come under intense pressure to reveal the exact details of the EU’s alleged threat. If it occurred in the way the government implies, the consequences for EU-UK relations may go beyond the Internal Market Bill. If Barnier or one of his team did threaten such action – presumably via legal action and sanctions – then that could blow up what is left of the talks.