Amber Rudd has warned "anything could happen" if Theresa May's Brexit deal is defeated in Parliament on Tuesday.
But work and pensions secretary Ms Rudd, one of the Prime Minister's key allies, warned Mrs May's deal was the best option available.
In a message to hard-line Brexiteers, she told the Today programme: "A lot of people have a perfect vision of what they think Brexit should look like, and that 'perfect' is not available."
It came as Iain Duncan Smith, a leading Eurosceptic and former Conservative leader, said Mrs May should quit if her deal is voted down by MPs.
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