I am told that the cabinet secretary Mark Sedwill and the attorney general Geoffrey Cox informed Cabinet that if at the end of the Letwin process MPs pass a motion mandating the PM to pursue a new route through the Brexit mess - perhaps a referendum, or membership of the customs union, or some other softer future relationship with the EU - the PM and government would be in breach of the ministerial code and the law if they fail to follow MP's instructions.
Or to put it another way, the PM would be obliged to endeavour to negotiate with the EU the revealed will of MPs, even if that revealed will involved a Brexit delay that requires the UK to participate in May's European parliamentary elections, or is at odds with the Tories' manifesto.
So the impression created by the PM that she could ignore the results of the indicative votes process is not true - or so ministers who attended Cabinet tell me.
And although the prime minister has said she would not negotiate a Brexit delay that obliges the UK to participate in those elections to the European Parliament, she would not have a choice - unless she quits.
This legal advice to her and her cabinet may explain why she is working to a new plan, to bring her own deal back to MPs for a third meaningful vote on Friday - because with the Letwin process starting tomorrow, she can now warn her MPs and Northern Ireland's DUP that they may genuinely face a choice between her deal and either no Brexit at all or a Brexit that is even less properly Brexity than her own version.
I imagine she will present this message to her MPs, when she meets them tomorrow at the 5pm meeting of the 1922 meeting (when she may or may not give a timetable for stepping down, according to different members of the government).
Whether enough of the ERG Brexiters among them will be scared enough of losing Brexit altogether to vote for her deal at the third go, well I slightly doubt that - but who can be sure?
Robert Peston is ITV's Political Editor. This article originally appeared on his ITV news blog.