Ken Clarke urges Tory rebels to ignore threat that customs union defeat could topple Theresa May in 'national interest'

Stakes raised by Downing Street hint that showdown will be a 'confidence vote' – meaning defeat would bring down the government

Ken Clarke has urged Tory MPs to ignore an extraordinary warning that defeat on the customs union would topple Theresa May and vote for the “national interest” regardless.

No 10 has piled pressure on potential Conservative rebels by suggesting next month’s showdown will be a “confidence vote”, the BBC reported – meaning defeat would bring down the government.

Such a threat would hugely magnify the importance of the vote – on whether the UK remains in a customs union with the EU after Brexit – and make it harder for Tory MPs to defy the prime minister.

But Mr Clarke accused No 10 of plunging into a “panic of the day”, after Brexiteer MPs demanded that Ms May hold the line, urging fellow pro-EU Tories not to be deflected by the hardline tactics.

“In the end, parliament has to decide what is in the national interest,” the former chancellor said.

The veteran Europhile pointed to the threat of “huge customs barriers and delays” and mounting evidence that leaving the customs union would force border checks in Northern Ireland.

An “absolute red line is we should not break the Good Friday Agreement”, which should not be dismissed as an “inconvenience”, he told BBC Radio 4’s Today programme. “It’s the next big problem and it will need all this to be resolved over the next month or two because it’s got to be – we won’t make much progress until the Irish problem is solved,” Mr Clarke said.

Meanwhile, CBI president Paul Dreschler issued a strongly worded rebuke to Sajid Javid, the communities secretary, after he insisted Britain must leave the customs union.

“An MP of your talent should rise above ideology and lead based of fact, analysis and evidence – all of which favours a customs union. Always happy to discuss,” Mr Dreschler tweeted.

Labour seized on the hints that votes on the customs union could be a “matter of confidence”, pointing that No 10 appeared to rule that out as recently as February.

But Jeremy Hunt made clear the government would not give way, saying: “The prime minister has been totally clear right from the outset. We’re not going to be part of the customs union or a customs union.”

Tensions rose over the weekend, after a Downing Street aide told The Sunday Times that Ms May and her allies “would not be crying into our beer” if parliament forced the government to back down. The scenario was said to have been “war gamed” at a recent meeting, with the conclusion that only Boris Johnson and Liam Fox would resign.

The controversy will come to a head after a heavy defeat for the government in the Lords last week. There will be a non-binding vote in the Commons on Thursday – with a full showdown next month.

At least 10 Tory rebels are ready to join with opposition parties to force a climbdown, potentially enough to defeat the government.