The inexhaustible riches of Geoffrey Cox’s advocacy poured over the Tory benches as he opened for the Government on the final day of the Brexit debate.

Here was a lost cause worthy of the Attorney General’s powers. He boomed, he declared, he pleaded, he went quiet for a moment, he turned again and again to face the Conservative benches, he jabbed his finger at his opponents on his own side, told them not to behave like children, lauded compromise as if he were Moses leading the chosen people through the wilderness towards a land flowing with milk and honey.

Beside him sat the Prime Minister. She looked white with exhaustion, mournful, almost hopeless. Yet during the hour he spoke, she revived like a wilting pot plant rescued at the last moment by a drink of water.

Cox opened by praising “the most passionate appeal to understand the role of compromise” voiced at midnight last night by the Member for Gedling – a Labour MP, Vernon Coaker, who according to Cox had been “heartfelt and eloquent”.

So the Government still hopes it can get its motion through with the help of Labour moderates. That at least was what Cox appeared to imply.

But as Rachel Reeves complained from the Labour benches near the end of this performance, for most of the time Cox turned to address his own party. Even the Speaker, John Bercow, asked the Attorney General to address the House rather than the Conservative Party: “This perambulation is very uncommon and irregular.”

“You upbraid me entirely justly,” Cox replied. But for the rest of the time, he did the upbraiding: “What are you playing at? What are you doing? You are not children in the playground, you are legislators.”

And as legislators, they must understand it would be “the height of irresponsibility” to pull the rug from under anyone who needs legal certainty, and can only get it if Parliament accepts the procedure for leaving the European Union which the Government has negotiated.

The Attorney General offered the curious analogy of an air lock, which we must enter in order to adjust our bodies to the different pressure we shall find when we pass through the second door on the far side and begin life outside the European Union.

Hilary Benn suggested, from the Labour benches, that beyond that second door lies “a complete vacuum”. Cox insisted on the contrary that we would find a “bright new world”.

But he offered another analogy. Removing ourselves from the EU is “as if we were to separate from a living organism with all its arteries and veins”.

It is a dangerous and complicated operation, about which we must be wholly pragmatic: “Do we opt for order or do we choose chaos?”

We cannot hurl the one million British citizens living on the continent of Europe, and the three million Europeans living here, “into a legal void”.

If MPs vote down the motion, “the path to Brexit becomes shrouded in uncertainty…and because of the Northern Ireland backstop”.

Cox had done his best to make rejecting the motion merely because of the backstop seem absurd, dangerous and disproportionate. When he realised he was in danger of going on too long, he quickly and skilfully brought his remarks to a close.

He had at least managed to cheer up the Prime Minister. Indeed, with this bravura performance, he had cheered up many people who are heartily sick of the whole Brexit debate.

But for him to need to speak with such force suggested also how desperate the Government’s predicament has become.