According to The Telegraph deputy political editor, Steven Swinford, on Twitter

And with that, cable is dragged down to a low of 1.3001 on the day. The headache just gets worse for Theresa May at this point. Swinford tweets that there is 40 Tory MPs ready to support the notion and if it passes it will effectively make the EU's backstop illegal - and that will only serve to cause further stall in negotiations between the UK and the EU.