Another Conservative revolt will force the prime minister to present her “plan B” within just three working days of what seems certain to be a heavy defeat of her proposed deal next Tuesday.
The victory torpedoed Ms May’s apparent plan to force MPs to vote multiple times on that deal, while “running down the clock” to the threat of crashing out of the EU with no agreement, as the feared alternative.
It triggered chaotic scenes in the Commons, as furious Brexiteers accused John Bercow, the Commons speaker, of blatant bias in allowing the vote, against legal advice.
The government must now table a motion, setting out what it plans to do if it loses on Tuesday, by 21 January – which, crucially, could be amended to allow parliament to “take back control”, as one senior Tory put it.
All eyes will be on the Labour leader, amid a growing expectation he will table a vote of no confidence in the government next week, to try to force a general election.
That is likely to fail, piling enormous pressure on Mr Corbyn to then back a fresh referendum – without which a Commons majority for a public vote is unlikely.
Chris Leslie, a Labour MP, said referendum supporters should act quickly, telling The Independent: “Now is the time to decide. Monday 21 could be parliament’s last chance to avoid the cliff edge. The decision on a people’s vote can’t be put off any longer.”
Sarah Wollaston, a leading Tory Final Say supporter, said it was time for the prime minister to concede a fresh referendum on Brexit without delay.
“Because parliament has reached an impasse, the right thing for her to do is say ‘parliament can’t decide, I’m going to take it to the people direct’,” she said.
But, by 21 January, there may still be no Commons majority for any alternative to Ms May’s doomed plan, whether a referendum, extending Article 50, or the soft Brexit “Norway option”.
Furthermore, even if an amendment did pass, the government would not be legally required to comply with what the Commons had voted for – although MPs would hope that political force would be sufficient.
Paul Williams, a Labour MP who has pledged to table an amendment for a new referendum at some stage, said it was too early to say if that would happen on 21 January.
He said: “I hope Labour moves a vote of no confidence as soon as May’s deal voted down and, if that doesn’t succeed, moves quickly to supporting a public vote on the final deal.”
MPs were divided over the significance of the defeat, on an amendment tabled by Dominic Grieve, the former Conservative attorney general, by a majority of 11.
Mr Grieve himself warned the government would not survive for long if it defied parliament and refused to listen to MPs’ instruction to “wake up”.
“It is, in a sense, parliament exerting and taking back control, doing what its job is there to do – which is holding the government to account,” he said.
“At the moment, the risk we have is that the government has been kicking the can down the road. It’s wake up time – we have a very short time in which to sort this out.”
But Jacob Rees-Mogg pointed out it did not have legal force, saying: “Parliament can pass motions saying the moon is made of green cheese, but that doesn’t mean it’s legally binding.”
The fresh setback for the prime minister came as she put herself on another collision course with Brussels, after toughening her stance on the Irish border backstop.
The government accepted a backbench amendment that could give the Commons a veto over entry into the backstop – even though it would be legally bound to do so, under the terms of the withdrawal agreement.
Furthermore, if the backstop is implemented, the government would have “a duty to have an agreed future relationship or alternative arrangements” with one year, so it “ceases to apply”.
Both contradict the divorce deal, under which the UK agreed the backstop was the default option if a trade deal is not in place by the end of 2020 and that it should apply indefinitely.
The move risked the EU’s wrath while being clearly designed to win over Tory MPs posed to vote down the prime minister’s deal next Tuesday.
But it was immediately dismissed by one leading Brexiteer, former minister Steve Baker, who said: “This flimsy rubbish will only persuade those who have decided to be persuaded. Most of us have made our decision.”
Until the Grieve defeat, Ms May was only required, after losing on her deal, to make a statement on what will happen next within 21 calendar days and then allow a vote in the Commons within seven sitting days.
That subsequent motion was not expected to be amendable until Mr Grieve’s victory, last month, which meant MPs would be able to put forward their alternatives – but only for indicative votes.
Before the defeat, Tory MPs raged at Mr Bercow for more than one hour, raising numerous points of order to vent their fury, in the closest the Commons comes to a riot.
David Natzler, the Commons clerk and expert on parliamentary procedure, was believed to have advised Mr Bercow that the Grieve amendment should be struck out, because it clashed with standing orders.
If the government does table the required motion within three working days, it could come very late on 21 January, postponing any debate until the following day.
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