Although Theresa May's Cabinet has finally managed to reach a loose agreement on what they would like to achieve from the upcoming negotiations, the Prime Minister's troubles look set to continue for the foreseeable. With Jeremy Corbyn expected to confirm that his party backs the UK staying in 'a' customs union post-Brexit, there's growing concern in Whitehall that May's government could collapse this year.

The most imminent threat comes from the Remain side of her party. The Sunday Times reports that Julian Smith, the chief whip, told May there is a 'very real threat' that Labour could unite with 15 to 20 Tory rebels to defeat the government on their decision to rule out membership of a customs union. This vote – on Anna Soubry's amendment to the trade bill – has been delayed to give the government time to try and remedy the situation. Karen Bradley, the Northern Ireland secretary, is said to have warned May that the DUP should not be relied upon to back the Prime Minister.

Meanwhile, David Davis is reported to have expressed concern that Jacob Rees-Mogg's appointment as leader of the European Research Group – the Brexiteer wing of the party – has led to MPs 'militarising' against May. Were May to do as the likes of Soubry wish and back 'a' customs union', this group could well submit letters demanding a leadership contest.

It follows that May is caught between a rock and a hard place. It's clear there's no Brexit position that will please her whole party. The question that remains: is there a form of Brexit that can keep her premiership alive? There's already talk that No 10 could make the vote on the customs union amendment into a confidence issue, threatening a general election if Tory MPs vote with the opposition. As James writes in the Sun, the thinking goes that the best way to persuade the rebels to back down is to make them realise that the government will fall if they back this amendment. Are these Tories really prepared to risk Prime Minister Corbyn or a general election with their party in total disarray for the sake of ‘a customs union’ with the EU?

It's hard to see how the majority could consider taking this path. Conservative MPs stood on a manifesto in the 2017 snap election that promised to take Britain out of the customs union. To risk bringing down the government over its attempt to honour a manifesto pledge seems problematic at best. The chaos that it could give way to would be seen as unforgivable by many. That's not to say that a handful of MPs wouldn't do it. MPs such as Anna Soubry feel isolated within their party since the referendum. Meanwhile Ken Clarke's EU views are so embedded it seems unlikely he could ever be convinced otherwise. If they believe that defying the government on Brexit is in the interest of the UK they may decide to put country before party. Likewise, some Brexiteers may make this calculation if the negotiations appear to point to the UK having 'a Brexit in name only'.

It follows that the government must work to try and ease concerns across the party over its Brexit position. Part of this is having a clear vision. That's been lacking for some time now – much to the chagrin of Remainers and Leavers. Now that the government has finally agreed an approach, the Prime Minister must articulate it. When May gives her big Brexit speech on Friday she doesn't just need to tell Brussels what she means, she also needs to convince her party of her plan. Without that, talk of a government collapse will only get louder.