London (CNN) UK Prime Minister Theresa May insists she "will not be derailed" from her duty to deliver Brexit, despite her government suffering an embarrassing defeat in the House of Commons last week.

Lawmakers last week backed an amendment to the Brexit bill, meaning the UK Parliament must be given a vote on the final deal with the European Union before withdrawal begins.

But May, writing in two UK newspapers, the Sunday Telegraph and the Sunday Express , argued that "last week marked a watershed" for exit negotiations when European leaders agreed to move on to the second stage of departure talks . She also vowed to prove her doubters wrong.

May has had to battle bitter divisions both within her own party as well as pressure from opposition parties and business leaders as she seeks to negotiate Britain's departure from the EU.

In her opinion pieces Sunday, she also addressed recent criticisms of her minority Conservative government's approach to the Brexit process.

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