Prime Minister’s Questions is no longer as central to British politics as it used to be. I remember Tony Blair once saying that if an MP wanted to keep something secret, they should say it in the House of Commons – preferably in a dull debate about training and skills. Now, it seems, you can say important things in the main event of the parliamentary week and no one will notice.

One of the most interesting things said this week was in one of Jeremy Corbyn’s questions to the prime minister. He asked Theresa May if she would confirm the “legal advice given to cabinet that, in the event of no deal, the government would still have to pay the EU a divorce bill of £30bn”.

It was an odd question, based on a report in The Daily Telegraph, in that her answer was utterly predictable. I may even have muttered it to my neighbour in the press gallery as she said it: “This is a country that honours its legal obligations...”

The significant bit, though, was Corbyn’s response. He has developed the habit of commenting on each of the prime minister’s answers before going on to ask his next question. Normally this is just time-consuming and irritating, but this time he said something new: “Last week, 63 Conservative MPs wrote to the chancellor to complain that Treasury forecasts based on Brexit negotiations are too negative. I am just waiting for them to write to say that the legal advice is too negative as well.”

This was a rather subtle point for the simplified, ritual exchanges of PMQs, but Corbyn was suggesting that the government’s legal advice was too cautious, and that, if the UK left the EU without an agreement, we should not pay as much as the bill that has already been agreed.

This is a familiar theme of the further reaches of hard-Brexit commentary: that, if we walk away from the EU without a deal, it can whistle for its money. The founding text of this sect of the Eurosceptic religion was the House of Lords EU Economic Affairs Subcommittee report in March 2017, which said that, in the event of a no-deal Brexit, “the UK would be subject to no enforceable obligation to make any financial contribution at all”.

What did Corbyn mean by his response to May? Was he positioning Labour for a no-deal scenario, where parliament has been unable to prevent our disorderly departure from the EU, so that he could argue – with nothing else left to oppose the government about – that we were paying too high a price?

Perhaps the line was suggested by Keir Starmer, the shadow Brexit secretary and former director of public prosecutions. As a lawyer, Starmer presumably takes a robust view of such matters. The best ministers, in my view, are those who refuse to accept legal advice as holy writ and are prepared to challenge advice that could be wrong.

All the same, it is a strange hill on which to choose to fight. As the Lords committee report went on to note, if the UK repudiated its financial obligations to the EU, it would “damage the prospects of reaching friendly agreement on other issues”. It would also damage the UK’s reputation as a reliable partner in international agreements.

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Maybe Starmer intends this stance only as a legalistic quibble over the amount payable, rather than the principle of paying anything. But I suspect that Corbyn, or his PMQs briefing team, meant more than this.

Anyone who is hoping, in the event of the government failing to strike a Brexit deal, that Corbyn would put Labour behind a campaign to keep the UK in the EU is not paying attention. Today, on the day of the march for a new referendum, Corbyn put out a statement about workers’ rights and Brexit: “We are determined to secure stronger rights at work and fair rules for business as we leave the EU as part of a new economic settlement which delivers for the many, not the few.”