Britain's opposition Labour Party is ramping up the pressure on UK Prime Minister Theresa May, saying it will call a no-confidence vote in the Conservative leader if Parliament rejects her Brexit deal on December 11.

May is battling to persuade sceptical British lawmakers to back the deal agreed between her government and the European Union last month.

Rejecting it would leave the UK facing a messy, economically damaging "no-deal" Brexit on March 29.

Labour Brexit spokesperson Keir Starmer said on Sunday it's "inevitable" that Labour will bring a no-confidence motion in the government if parliament rejects the Brexit agreement.

The left-of-centre party says it will vote against the deal.

If May's Conservative government lost a no-confidence vote it would have two weeks to overturn the result. If that failed, Britain would hold a new national election.

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