Theresa May will be forced to come up with a Brexit ‘plan B’ within three days if her deal is voted down on Tuesday, after suffering another humiliating Commons defeat. 

It is the second setback faced by the prime minister within 24 hours, inflicted by a cross-party group of MPs who want to block a no-deal Brexit.

Crucially, it will allow MPs to table amendments to whatever motion May brings back, allowing the will of the Commons to be tested on Brexit ‘plan Bs’ such as a second referendum or a Norway model much sooner.

The PM will be forced to return to the Commons within three sitting days of losing, suggesting a date of Monday January 21.

The government initially felt it had seen off the latest “guerilla warfare” plot from the rebels, because it thought the amendment tabled by former cabinet minister Dominic Grieve would not be allowed under Commons rules.

But Commons Speaker John Bercow sparked fury from Tory MPs by allowing the plan to be put to a vote, which the government subsequently lost by 308 votes to 297, majority 11. 

Downing Street had tried to see off the Grieve amendment by promising the PM would “seek to provide certainty quickly” if she loses the vote on her under-fire deal next week, as expected.

But the amendment will force the PM on to the rebels’ timetable, and bring her deal back in front of MPs for a second time after just three days – barely any time to get extra concessions from Brussels.  

The group led by Grieve feel they must speed up the parliamentary process to stop May running down the clock to exit day on March 29 in an attempt to pressure MPs into backing her deal to guard against no-deal.

Despite the impasse over Brexit in parliament, Bercow’s move to allow the amendment sparked utter fury from both Tory hard Brexiteers and those backing May and her deal.

A Number 10 spokesman said: “We are surprised that the Grieve amendment was selected, the advice we received was that it would not be in order. We have access to resources of the House of Commons as well as our own.”

But facing more than an hour of points of order in the Commons on Wednesday, Bercow stood by his decision.

“My understanding is the motion is amendable, I’m clear in my mind about that,” he said.

After heckles from the Tory benches, he added: “I’m trying to do the right thing and make the right judgments. That is what I have tried to do and what I will go on doing.”

Tory former minister Mark Francois, deputy chairman of the Eurosceptic European Research Group (ERG), claimed Bercow was not following his own rules.

Francois read the government’s original business motion, which states that additional motions are not allowed.

However Bercow insisted an amendment, like Grieve’s, was different to a motion, provoking Francois to begin yelling “ridiculous” and “that is utter sophistry” as the Speaker defended his decision.

“The answer is simple,” Bercow said.

“He referred to a motion and he said that no motion in this context, for the purposes of precis, may be moved other then by a minister of the Crown. ’Tis so.

“We’re not speaking here of a motion but of an amendment to a motion. I’m sorry but there is a distinction between a motion and an amendment.

“What he says about a motion I accept but it doesn’t relate to an amendment. That is the answer.”

Commons leader Andrea Leadsom noted there were “some concerns” about Bercow’s decision and challenged him on claims he had overruled the most senior Commons official, clerk Sir David Natzler, in making his decision.

She asked him to publish Sir David’s advice, prompting cheers from Tory MPs following last month’s row over the government’s Brexit legal advice.

Bercow confirmed he consulted the clerk and officials, saying the advice was given to him “privately and that’s absolutely proper”.

He said: “It’s also true I had a written note from the clerk from which I quoted in responding to the first point of order.”

Tory MP David Morris leapt to his feet and repeatedly shouted: “Publish it!”

Conservative grandee Ken Clarke said he found it “unbelievable” that some MPs were trying to stop the Commons expressing its opinion and suggested those who were angry should join the mob of yellow-jacketed Brexit supporters causing controversy with their protests outside parliament

He said: “I would suggest to some of my honourable friends, the ones that are getting somewhat overexcited, that perhaps they should don a yellow jacket and go outside.”

Tory MP Heidi Allen meanwhile branded the entire Commons out-of-touch.

“I am absolutely hopping mad,” she said.

“When I became an MP three years ago I was determined that I would not become part of the establishment. Do people in this House have any idea how out-of-touch the general public think we are most days?

“We are talking about 79 days until potentially crashing out of Europe without a deal – should our focus not be on the detail and the arguments about the process in this place, but getting on with a plan B if Parliament decides the government’s plan is not the one for the people?

“When are we going to start acting like public servants doing the right thing and having the debate and getting on with it?”

Nikki Da Costa, former director of legislative affairs at Number 10, said the speaker had, by selecting the amendment, overturned the procedure set out and approved by the Commons in the EU Withdrawal Act (EUWA).

She tweeted: “More than anything this is not just overturning something the House accepted in a business motion, it is overturning the EUWA and procedure voted on and approved in both Houses with much debate and scrutiny, and indeed which Grieve accepted on floor of the House.”

The latest parliamentary fireworks come after the anti no-deal MPs defeated the government on Tuesday evening, in a significant show of parliamentary strength against what they feel would be a “cliff-edge” withdrawal from the EU.