At last, Labour steps up. Brexit is the great national crisis of our times and yet the leaders of the opposition have sometimes seemed so muted it has driven remainers to tear their hair out in frustration.

That changed yesterday at prime minister’s questions. Jeremy Corbyn for the first time turned all guns on the prime minister over her incoherent, incomprehensible and impossible Brexit stance. He used all his questions, every one, to wallop her exactly where she and her party are most vulnerable – and not before time.

If ever there were an open goal, it is the warring party of government whose demented 40-year obsession with the EU is finally driving us all to destruction. On budget day, PMQs may get less attention than usual, but the signs are that Corbyn’s blistering salvos are just the opening shots in what should be a weekly cannonade.

Here’s his best tirade: “Seventeen months after the referendum they say there can be no hard border but haven’t worked out how. They say they’ll protect workers rights, then vote against it. They say they’ll protect environmental rights, then vote against it. They promise action on tax avoidance but vote against it time and time again.”

The prime minister’s lame response was: “Let me tell him, I am optimistic about our future. I’m optimistic about the success we can make of Brexit … blah, blah, blah … building a Britain fit for the future …” Naturally, anyone who questions her nonexistent post-Brexit plans or visions is a traitor, “talking down Britain”.

Half an hour later as the chancellor rose, there was no way he could avoid talking down Britain, no sugar to coat the pill as he read out dire budget figures showing growth and productivity falling further behind the Europe many Tories so despise. And Brexit is the reason.

As the true meaning of the forecast sinks in, everyone sees the longest fall in living standards for 50 years, lasting until the middle of the next decade. And even then, who knows?

Where has Corbyn been? On a journey, say those close by. A lifetime of instinctive “capitalist club” Euroscepticism has been shed. Passionate distress over Brexit from his young supporters and his trade union allies has brought him round. Besides, the facts have changed. His vague, abstract distaste for the EU has given way to facing the hard reality of what Brexit means: inflicting most harm on those he cares about most. If only those on the opposite benches were on the same reality-check journey.

In PMQs he has usually dodged the great issue. But his tone changed recently: on a visit to Shipley, in West Yorkshire, he was asked how he would vote if there were a referendum now – a question the PM and chancellor duck, while those turncoats Jeremy Hunt and Liz Truss cravenly recant. Corbyn unhesitatingly said he’d vote remain: “I voted remain because I thought the best option was to remain. I haven’t changed my mind.”

He added: “We must make sure we obtain tariff-free access to the European markets and protection of all the rights and membership of agencies we have achieved through the European Union.”

He was, say some, hesitant on unfamiliar policy turf. But now he has found his feet, and his voice and confidence. “The danger is, we will get to March 2019 with no deal, we fall out of the EU, we go on to World Trade Organisation rules, and there will be threats to a lot of jobs all across Britain,” he warned. “I think it is quite shocking.”

This time next week the EU’s chief negotiator, Michel Barnier, will deliver his verdict on whether sufficient progress has been made. He has already thrown down the challenge to the chaotic Tories: what kind of country does Britain want to be, a European model country, or something else altogether?

Theresa May doesn’t know, but Jeremy Corbyn does. The European model beckons as the enlightened, internationalist, progressive vision – the Europhobic model is a land of impoverished deregulation.

There were obvious reasons for Labour’s reluctance to go full-tilt against Brexit. Too many Labour MPs in leave seats had taken fright. But since the election, another picture has emerged: Labour lost votes in some leave seats but gained votes in other leave areas as electors lost faith in the government’s chaotic negotiations.

Keir Starmer, the shadow Brexit secretary, has led the way, opposing every government misstep, aligning maximum opposition amendment by amendment. His leaders cannot but see that this is not just right, but politically essential. There is no other place for an opposition to be in this national trauma. My hunch is that the harder Corbyn hits out over Brexit, the stronger Labour’s support will grow. And the word is, that’s what we shall hear from now on.

• Polly Toynbee is a Guardian columnist