The former attorney general has launched a scathing attack on those who want Britain out of the EU.
Dominic Grieve warned that with the international situation so precarious there were significant dangers to leaving a supranational organisation such as the EU.
He added that the free movement of people within was clearly non-negotiable.
"The requirements of any free trade agreement would make British removal from the clauses dealing with freedom of movement impossible", Grieve said.
"With the curious consequence that the single biggest cause of domestic irritation with the EU, immigration, would remain unaltered. But without its maintenance some two million UK citizens working in EU countries would find themselves becoming illegal immigrants overnight".
Grieve has long been an opponent of the most hardline elements of Euroscepticism within the Conservative Party.
He has supported Britain's membership of the European Convention on Human Rights, which many of his Tory colleagues would like to see replaced with a British Bill of Rights.
Grieve argued today:
The success of the convention, despite all its shortcomings, in raising standards of behaviour and promoting human rights globally, and with it the overall security of the Europe, is to be disregarded for the sake of addressing irritations about some of its current domestic impacts which at best will be of utterly marginal benefit.
His remarks, reported by the Guardian, shed light on the feelings of the handful of pro-EU Tories. Grieve was, to say the least, sceptical of his party's plans for a renegotiation of membership terms followed by an in-out referendum.
Today he added:
There is... a total lack of clarity as to how a government would proceed to unravel a relationship that has developed in complexity over more than 40 years.
Grieve went on to say that he was still none the wiser about which parts of the "several thousand pieces of EU legislation" would be incorporated into UK law following Brexit.