A grey dusk had already swept in from the Thames on Thursday, cloaking the Victorian gothic battlements of the Houses of Parliament, when Theresa May’s team learnt about the plot against them.

Julian Smith, the Conservative chief whip, stopped briefly in the members’ cloakroom to drum up voters for the prime minister’s Brexit deal, which will come before MPs on Tuesday.

Like so much in the Palace of Westminster, the room is an eccentric throwback to the 19th century. Purple ribbons still hang from each coat hanger, where in times past MPs would hang their swords. There, among the overcoats, Smith overheard a senior Tory backbencher excitedly discussing a plan that had been hatched for parliament to seize control of the Brexit negotiations.

For weeks,