Senior ministers were invited to review the text of the withdrawal agreement that has so far been secured in negotiations with Brussels, and are poised to meet as soon as a deal is ready to be signed off.
"Repeatedly people seem to make the same mistake over and over again, assuming that if the British cabinet agrees something, well, then that's it then, everything is agreed".
But he rejected the idea Britain could unilaterally end the arrangement, something Brexit Secretary Dominic Raab and other eurosceptics in May's Conservative party want.
Liam Fox has said the decision as to whether to end the Brexit backstop has to be for the United Kingdom alone - in a challenge to Theresa May.
Any version of the backstop would apply unless and until a wider UK-EU deal on the future relationship solved the issue of how to avoid a hard border with Ireland.
Sky News understands Environment Secretary Michael Gove, a leading Brexiteer like Dr Fox, is also among those cabinet ministers wanting to view Mr Cox's advice in full before giving their consent to a deal.
Earlier today former Brexit secretary David Davis said it was "not acceptable" to make the United Kingdom wait until the Irish government and EU Commission had given it approval to withdraw from the backstop. It appears the Prime Minister is wedded to the idea of a border down the Irish Sea with Northern Ireland in the European Union single market regulatory regime'.
The European Union's fallback proposal aimed at avoiding a hard border between Ireland and the United Kingdom would effectively keep Northern Ireland aligned with Brussels's customs union and single market.
May will still need to get the deal, including the so-called Irish border backstop plan, approved by her Cabinet and later by Parliament - where she's likely to face considerable opposition.
A potential sticking point could be demands for European Union fishing fleets to be given continued access to British coastal waters as the price for agreeing to Mrs May's UK-wide backstop, the Daily Telegraph reported.
The European Union's chief Brexit negotiator told political allies Thursday to beware the rise of populist nationalism in Europe, warning of a "Farage in every country".
Speaking to a conference of the centre-right European People's Party (EPP) in Finland, Mr Barnier said: "The European project is fragile, it is under threat, it is perishable and at the same time it is vital".