The Opposition ought to be taking the Conservatives apart. This is the view of someone in government – someone who has a high opinion of Theresa May, but who told me this week: “A Blair or Cameron-led opposition, let alone many of the oppositions of the Sixties and Seventies, would possibly bring the Government down this autumn.”

We have had two hints of how a more effective opposition might operate. First, when Emily Thornberry stood in for Jeremy Corbyn at Prime Minister’s Questions in July. She quoted what different ministers had said about the possibility of the UK and the EU failing to reach agreement on Brexit – “perfectly OK”, said Boris Johnson; a “very, very bad outcome”, said Philip Hammond.

This is one of the most effective lines of attack available to an opposition leader. Tony Blair deployed it against John Major in 1995 about the government’s policy on the euro: “I find it odd that he cannot agree with his Chancellor, I find it strange that he cannot agree with his Secretary of State for Employment and I find it unbelievable that he cannot agree with himself.”

Thornberry seemed to be enjoying herself, which may be one reason Len McCluskey, the leader of Unite, Labour’s biggest affiliated union, tipped her as a possible successor to Corbyn. Corbyn rarely mentions Brexit at PMQs, and he also doesn’t think it is his job to comment on internal Tory differences, such as when Iain Duncan Smith resigned as Work and Pensions Secretary last year.

'No decisive progress' on Brexit, says EU negotiator Michel Barnier

Corbyn’s reluctance to talk about Brexit may have been an astute judgement, however, given his personal ambivalence about the EU and the large number of Labour voters who voted to leave in the referendum. He certainly held together a remarkable coalition in the election of Labour leavers and people who voted Labour because it was the best chance of stopping Brexit.

But now we come to the second hint of what an effective opposition might look like, with the new policy set out by Keir Starmer, the Shadow Brexit Secretary, last weekend. His plan for a long transition period – four years – during which Britain is outside the EU but still inside the single market, like Norway, is calculated to maximise Theresa May’s discomfort.

The Government published several Brexit position papers over the summer, and the consistent theme of all of them is to try to keep things as much the same as possible. Why not, therefore, stay in the one bit of the EU that was a British priority in the first place and that makes us richer? The answer to that is that Labour wants to end free movement of labour, but as a temporary phase the single market is a clever compromise.

Once again, it allows Labour to appeal to leavers – Brexit is happening and free movement will end eventually – and to remainers – we keep most of the benefits of EU membership for a while, and maybe even permanently. Tom Watson, the deputy Labour leader, said on Thursday that the single market “might be a permanent outcome of the negotiations”. What he avoided saying was that permanent single-market membership would need the rest of the EU to agree to restrict the movement of people, which is most unlikely, but Labour is not in power so its proposals are not held to the same standard of plausibility as the Government’s.

That is the secret of really effective opposition. It does not have to be entirely responsible or realistic. The best example might be not Blair or Cameron but John Smith, Blair’s predecessor as Labour leader, who harried John Major’s government mercilessly over the last great European negotiation, the Maastricht Treaty in 1992-93. (David Davis was then the junior government whip responsible for the bill.) Smith was a paleo-pro-European, having broken the Labour whip with Roy Jenkins in 1971 to vote in favour of joining the EEC in the first place. But he took the risk of killing the Maastricht Treaty by making common cause, cynically, with Tory Eurosceptics.

Brexit: the deciders Show all 8 1 /8 Brexit: the deciders Brexit: the deciders European Union's chief Brexit negotiator, Michel Barnier Getty Brexit: the deciders French President Emmanuel Macron Getty Brexit: the deciders German Chancellor Angela Merkel Reuters Brexit: the deciders Commission President Jean-Claude Juncker EPA Brexit: the deciders The European Parliament's chief Brexit negotiator Guy Verhofstadt Getty Brexit: the deciders Britain's Prime Minister Theresa May Getty Images Brexit: the deciders Britain's Chancellor of the Exchequer, Philip Hammond PA Brexit: the deciders After the first and second appointed Brexit secretaries resigned (David Davis and Dominic Raab respectively), Stephen Barclay is currently heading up the position PA

The government was defeated, and Major was forced to reverse it by calling a vote of confidence. If he had lost, there would have been an election, so the Tory rebels returned to the fold. Smith had not brought the government down, but he had inflicted further damage on Major’s reputation: he declared the vote was “not a sign of confidence, but a display of weakness”.

We are in a similar position today. Starmer has written to Davis threatening a series of amendments to the EU (Withdrawal) Bill, although I doubt that Labour can defeat the Government on the question of staying in the single market for a transitional period – it would need 15-20 Conservative MPs to rebel. But someone of Smith’s ability could embarrass Theresa May in the Commons so badly that it would make the next election really hard for the Tories.

Jeremy Corbyn has his qualities, as the last election proved, but forensic demolition in Parliament is not one of them. And who would have thought, when Corbyn’s Labour opponents criticised his support for the IRA, that it would become such a critical factor in parliamentary arithmetic?

In 1993 the unionists split, with the DUP voting against Maastricht but the mainstream Ulster Unionist Party voting with John Major. Who knows if a different Labour leader could persuade the deeply Eurosceptic DUP this time, but Corbyn’s history in Irish politics guarantees that its 10 MPs will stick by the Government.

Not even John Smith could bring the government down in 1993, but how much could Labour undermine the Government’s foundations this time? Brexit is going to be hard enough for the Government, and Theresa May’s poor ratings could suffer further as a result. But so far, in the Commons, the Labour leadership has gone along with the Government’s policy on Brexit.