Theresa May has warned there is a "very real risk of no Brexit" or elections if Parliament rejects her plan for Britain to leave the EU.
May's government is widely expected to lose a Tuesday vote in Parliament on her Brexit plan reached with Brussels last month, with the opposition Labour Party, the prime minister's nominal allies in Northern Ireland's Democratic Unionist Party (DUP) and some members of her conservative party saying they will not support the deal.
Read more: UK leadership challenge: How does it work?
It is unclear what would happen if the deal fails, but it could put Britain's Brexit plans in jeopardy and May's job on the line.
May told the Mail on Sunday newspaper that Britain "would truly be in uncharted waters" if the Brexit deal reached after two years of intense negotiations with the EU was rejected less than four months before the country is set to leave the block on March 29.
A failure of the Brexit deal to pass would "mean grave uncertainty for the nation with a very real risk of no Brexit or leaving the European Union with no deal," she said.
New elections or a conservative revolt against the government could also lead to Jeremy Corbyn's Labour Party coming to power, May warned.
EU: Deal cannot be renegotiated
Downing Street has insisted Tuesday's vote will go ahead amid calls from members of Parliament to renegotiate the Brexit deal with Brussels.
Read more: The draft Brexit deal: What you need to know
The EU has said it may consider "cosmetic" changes to the non-binding political agreement on future relations, but the legally binding 585-page withdrawal agreement cannot be renegotiated.
European Council President Donald Tusk said Sunday he had spoken to May by phone ahead "an important week for the fate of Brexit."
Pro-Brexit lawmakers argue the deal binds Britain too closely to the EU, while europhiles say it creates barriers between the UK and its biggest trading partner and casts into uncertainty the future relationship with the bloc.
The biggest stumbling block centers around the so-called "backstop," a provision designed to keep an open border between EU member Ireland and British-ruled Northern Ireland.
The temporary measure would keep Britain under EU customs rules until permanent new trade arrangements are made. But critics say it could tie Britain to the EU indefinitely, lead to Northern Ireland being treated differently and tie the UK's hands in striking trade deals with non-EU states.
Thousands protest in London
The Brexit debate has deeply divided the country. On Sunday, riot police were deployed in London as thousands of pro-Brexit protesters led by far-right activist Tommy Robinson were countered by pro-EU demonstrators.
Pro-EU lawmakers have pushed the idea of a second referendum on Britain's EU membership after the first vote narrowly passed in June 2016, but the government has opposed a new vote.
Meanwhile, EU supporters are hoping the European Court of Justice will rule on Monday that Britain has the right to unilaterally stop Brexit.
cw/xx (AFP, AP, dpa, Reuters)
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