Letwin amendment 'boosts chances of no-deal' - No 10
If the Letwin amendment passes, it is "too late" for the government to pull the main vote on the deal, says BBC political editor Laura Kuenssberg.
But it can be "negatived", she says - ie if the government doesn't provide tellers or no-one shouts "aye" on the second call then the vote could just come to a halt.
She adds that Downing Street is suggesting the Letwin amendment increases the chances of no-deal happening.
The Letwin amendment would provisionally approve Boris Johnson's Brexit deal, but withhold official support and delay Brexit until the necessary legislation is passed.
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BREAKING
MPs vote on Letwin amendment
The House of Commons is voting on independent MP Sir Oliver Letwin's amendment to the government's motion.
This would withhold parliamentary support for the government's deal unless and until legislation implementing the agreement in UK law is passed by MPs.
If it's passed, it could force the prime minister to seek a further delay to Brexit.