UK Prime Minister Theresa May faces a vote of confidence in her leadership later after 48 of her Conservative MPs called for one to be held.
But a defiant Mrs May vowed to contest the vote “with everything I have got”.
She warned that a new prime minister would be faced with the choice of “delaying or even stopping Brexit”.
A majority of Tory MPs have publicly said they will back the PM in the vote, which runs for two hours from 18:00 GMT, but it is a secret ballot.
A result is expected fairly quickly after the voting finishes.