U.K. Prime Minister Theresa May struck a defiant tone Thursday as she addressed lawmakers in Parliament following the resignation of two senior government officials, including her top Brexit negotiator, in the wake of a deal last night that sets out Britain's exit from the European Union. 

May told Parliament that her deal, the text of which was agreed by both Cabinet in London and official in Brussels after months of torturous negotiations, was the best possible arrangement for Britain as its prepares for its March 29, 2019 withdraw from the European union. Her remarks, which had been drafted following her Cabinet agreement last night, followed the resignations of Brexit Secretary Dominic Raab and Pensions Secretary Esther McVey -- as well as three other junior ministers --- and came amid increasing speculation that her Conservative party lawmakers will trigger a leadership challenge.

"We can choose to leave with no deal... risk no Brexit at all... or unite and support the best deal that can be negotiated," May told the House of Commons.

The pound fell sharply following Raab's resignation on fears that it could also trigger fresh national elections in the United Kingdom early next year. Sterling was marked 1.7% lower against the U.S. dollar at 1.2800 in London, after hitting the lowest level in 17 months, while European stocks reversed earlier gains and slipped 0.5% into negative territory.

Britain's FTSE 250, a broader benchmark for domestically-focused U.K. stocks, was marked 1% lower following Raab's decision, as Barclays' Plc (BCS shares slumped more than 5.6% and government-backed Royal Bank of Scotland (RBS shed 5.3%.

"The government is falling apart before our eyes as for a second time the Brexit secretary has refused to back the prime minister's Brexit plan," Labour's shadow Brexit Secretary, Jon Trickett, said in a statement. "This so-called deal has unravelled before our eyes."