Struggling to win parliamentary support for her Brexit plans – the negotiations that will define Britain's withdrawal from the European Union in March 2019 – Prime Minister Theresa May is facing her darkest hour.

The central issue this week is that May is trying to win a broad-outline deal with the EU in relation to its customs border with Northern Ireland. Part of the U.K., Northern Ireland also borders the EU member state, Ireland, so it either requires a physical border (think Mexico) that controls trade access with the Ireland, or a deal that allows EU trading rules to be applied in Northern Ireland. But Theresa May and the EU have unable to reach terms on what such a deal might look like. Making matters worse, as she addressed Parliament on Monday, May found herself under fire from all sides as to what she is willing to accept with the EU and how long any transition arrangement might last.

As things stand, May's Brexit plan cannot command parliamentary assent. But it gets worse. With time rapidly running out for the EU and U.K. to agree a deal – the EU wants a deal by Tuesday evening – May is trapped on all sides. What happens next?

May might change course in order to better consolidate herself with Conservative members of parliament who believe she is giving the EU too much. But that course of action makes it almost certain that the EU pulls out of Brexit negotiations and accepts a no-deal Brexit. The problem is that such a withdrawal would cut off privileged trading access to the EU and threaten Britain's export economy.

Alternatively, May might keep playing the waiting game with the EU in the hope that the trading bloc offers an improved deal that she can sell to Parliament. But if Labour Party leader Jeremy Corbyn senses that May cannot unify Conservatives around her government, he will attempt to destabilize the prime minister by blocking her Brexit plan and effectively forcing her to resign. That could even bring pressure on May's successor – perhaps Boris Johnson – to call an election.