U.K. Prime Minister Theresa May's ultimate showdown has finally arrived.



Her high-stakes parliamentary vote on her Brexit plans is due later on Tuesday and the outcomes vary wildly from soaring success to complete political collapse.



Currency traders lie in wait, hoping not to be caught on the wrong side of a sterling trade that could whip wildly in either direction as the debate rages and the votes roll in. It could be a late night in the City of London and thin trade from other regions of the world could exasperate matters.



"Sterling liquidity in Asia is limited and as such there is a risk that even if a no vote is assumed that does not preclude sterling selling off, impacted by headline risk," Jeremy Stretch, the head of G-10 foreign exchange strategy at CIBC Capital Markets, told CNBC via email.

The consensus expects defeat for May and her Brexit proposals but what's crucial is by how much. She needs the backing of 320 lawmakers, more than half of the 639 that vote in Parliament.