Theresa May faces a Commons showdown over attempts to stop her making the final decision on Brexit - and to put Parliament in charge instead.
In what promises to be a dramatic day at Westminster, MPs will vote on proposals to ensure they have a “meaningful vote” on any Brexit deal signed with the EU.
It would mean a Brexit deal can only go ahead if Parliament approves it.
Crucially, if Parliament rejects the deal - or if it simply proves impossible to come to an agreement with the EU - then Mrs May would lose control of the Brexit process.
Instead, the Government would be forced to carry out the wishes of the House of Commons and the House of Lords when it comes to future negotiations with the EU.
This would allow MPs Lords to insist that the Government re-open negotiations, even if it means delaying Brexit.
And they would set the “direction” of the negotiations, which could mean options such as staying in the EU single market or in the customs union come back into play.
The proposal is one of a number of amendments added by the House of Lords to the European Union (Withdrawal) Bill, the legislation that will take the UK out of the European Union.
The Government is asking MPs to delete the Lords amendment, and a vote is expected at around 4pm on Tuesday June 12.
Losing this would be a huge blow to Mrs May’s authority.
Labour will vote against the Government, along with other opposition parties such as the SNP and the Liberal Democrats.
The key question for Mrs May is whether they will be joined by enough rebel Conservative backbenchers to defeat the Government.
Tories including former attorney-general Dominic Grieve, former education secretary Nicky Morgan, former business minister Anna Soubry, and South Cambridgeshire MP Heidi Allen have backed similar proposals in the past.
Other votes expected to take place on Tuesday include a Government attempt to add the planned “exit date” of March 29 2019 back into the legislation, after it was removed by the House of Lords.
In a vote on Wednesday, the Government will attempt to overturn a requirement that the Brexit Secretary deliver a report on efforts to keep the UK in a Customs Union with the EU.
The Government will also attempt to delete a Lords amendment requiring the UK to attempt to negotiate a deal allowing it to participate in the European Economic Area (EEA) after Brexit day. In practice, this would mean staying in the EU single market.
This could be a difficult vote for Labour, because the party’s official position will be to abstain but many Labour backbench MPs believe the UK should stay in the single market, and are likely to rebel against the party leadership by voting against the Government.