British Members of Parliament are to vote later whether to give Theresa May the power to get Brexit negotiations under way. The government is expected to win, with most Conservative and Labour MPs set to back its European Union Bill.
But Labour leader Jeremy Corbyn faces a rebellion by some on his side, while the SNP and Liberal Democrats are also promising to oppose ministers. The vote, which will follow two days of parliamentary debate, is expected at about 19:00 GMT.
On Monday, politicians made impassioned speeches for and against the bill, which, if passed, will allow Mrs. May to trigger Article 50 of the Lisbon Treaty by her own deadline of 31 March.
This would get formal Brexit negotiations with the EU started, with the UK expected to leave the 28-member group in 2019.
Brexit Secretary David Davis said MPs had to implement a decision made by the people in last June's referendum, which the Leave campaign won by 51.9% to 48.1%. Doing otherwise would be viewed dimly, he warned.
Mr Corbyn has imposed a three-line whip - the strongest possible sanction - on his MPs to back the bill, which is only two lines long.
Shadow Brexit secretary Sir Keir Starmer called the vote a difficult decision for Labour - most of whose MPs supported Remain in the referendum - but it had to accept the result.
Two shadow ministers have quit Labour's front bench in order to oppose the bill, while MPs Stephen Timms and Lyn Brown told the Commons they would also vote against it.
A government source said up to 30 Labour MPs were expected to defy Mr. Corbyn.
If the vote goes the government's way, the bill will return to the Commons next week for the committee stage, when opposition parties will try to push through a series of amendments.
The bill was published last week, after the Supreme Court decided MPs and peers must have a say before Article 50 could be triggered. It rejected the government's argument that Mrs. May had sufficient powers to trigger Brexit without consulting Parliament.