Campaigners for a second EU referendum are gearing up for a crucial 72 hours if Theresa May’s Brexit deal is voted down, in which they hope to “kill off” the alternative Norway-style ‘plan B’ floated by some MPs.

May has promised to “move quickly” if her deal is rejected by the Commons on Tuesday, but it is unclear in what direction, and battle lines are being drawn as MPs try to put together a majority for their preferred alternative.

One Whitehall source said preparations are already underway in Downing Street for Labour to call a vote of no confidence in the government as soon as Wednesday, while another said the PM would be considering whether to respond immediately to the vote result with a point of order in the Commons, like David Cameron did after being defeated on military action in Syria in 2013.

The margin of defeat, the nature of the debate, the EU’s response and the reception from Tories to fresh reassurances from Brussels over the controversial Irish border backstop are all expected to influence the PM’s response.

But rival factions for alternatives to May’s deal are already gearing up for cross-party talks as MPs try and stitch together together a parliamentary majority ahead of potential “indicative votes” on different options later this month, with all sides expecting the debate to “move very quickly”.

A cross-party group of People’s Vote backers will meet on Wednesday morning at around 10am in Smith Square, around the corner from the Houses of Parliament in Westminster, to talk about common areas of agreement for the following 72 hours.

A senior and well-placed source predicted there would also be “furious lobbying of MPs”, with 10-15,000 voters being encouraged by People’s Vote campaigners to target MPs in 100 key seats, including key Tories, Labour waverers and potential opposition supporters of May’s deal, like Caroline Flint and Gareth Snell.

The source also said there would be a “releasing of dossiers” critical of the rival plan backed by some Tory and Labour MPs for Norway-style membership of the single market “to kill it off once and for all”.

But backers of both a second referendum and Norway backers are expecting more MPs to come out and publicly support their campaigns in the aftermath of a defeat for May.

Around 30 MPs including four or five Tories and some Labour frontbenchers are predicted to come out and publicly call for a People’s Vote, while Norway–option supporters also expect a string of new backers from both parties.

The source said they expected a second referendum to have around 180 backers by the end of next week and Norway to have 50-60, but acknowledged that Norway has the “surge capacity” to pick up momentum.

A senior Norway-supporting Labour MP said their option could gain the upper hand as it can command broader support among Tories.

They said a “sizeable number” of Labour MPs would vote against a second referendum “in any circumstance” while only a “really, really small number” Tories would back it.

The MP stressed that around half of the 20 Tory rebels who helped defeat the government last week are “very against another referendum, Sir Oliver Letwin types, Nick Boles and so on” and so it was difficult to see a majority for a People’s Vote.

 “The challenge for us in terms of our Common Market 2.0 or ‘Norway Plus’ proposals is getting enough oomph behind it in time,” the MP said.

“But the potential support for it is incredibly broad.

“What we’re keen to demonstrate is this solution is appealing to, and does appeal to, Leave and Remainers.

“One of the challenges with no-deal or the second referendum is they seem to be one camp or the other.”

If May insists on bringing back her deal Labour is likely to hold the key on forcing her into a potential plan B.

A well-placed source said the party was “not averse” to cross-party working, highlighting exchanges in the Commons this week between shadow Brexit secretary Sir Keir Starmer and senior Tories Sir Oliver Letwin and Ken Clarke on efforts to find a majority in parliament.

They acknowledged that the Norway model may once have won support within Labour but stressed that the party had identified problems with it, not least that the Scandinavian country itself would not support the UK having a similar deal.

Labour’s response will also depend on what May says after losing any vote. But the source said that the party is preparing to make daily decisions next week on whether to call a no confidence vote in the government if her deal is rejected by MPs.

Tory hard Brexiteers are meanwhile expected to underline their opposition to May’s deal by putting out an “alternative” written ministerial statement setting out plans for a looser free trade deal based on Canada and Japan’s agreements with the EU.

It will say the hated Irish border backstop can be dropped and replaced with a free trade deal on goods and a commitment from all sides that any necessary checks will take place away from the border.

The plan will also provide for a standstill transition period under the general agreement on tariffs and trade to give time to negotiate the trade deal.

Senior Leaver Steve Baker said: “We the Brexiteers know what we want and we know how to get it and it is very frustrating that we won’t get the opportunity to do it.

“We can rescue the position of the UK, the government and the Tories.”