Theresa May could trigger article 50, the formal process for leaving the European Union, as early as Tuesday if the Brexit bill passes late on Monday, government sources say.
The legislation, which passed through the House of Commons unamended, is due to be debated by MPs on Monday. They will have to decide whether to accept a pair of amendments added by peers – on the rights of European Union citizens and granting parliament a meaningful vote at the end of the process.
Rebel Conservative MPs have urged the Brexit secretary, David Davis, to give firm promises on both issues when he addresses the Commons.
Anna Soubry, who has been one of the most outspoken rebels, said she was keen to be reassured that MPs would be given a vote in the event that the government is unable to secure a deal in the two-year negotiating period.
“It’s completely unacceptable that in the event of no deal, the people of this country, or their elected representatives, their MPs, would have no say on what happens next,” she said.
The shadow Brexit secretary, Keir Starmer, and Labour’s leader in the Lords, Baroness Smith, wrote to the prime minister on Friday asking her to act on both issues.
“We would urge you to reflect and reconsider on the overwhelming case to act on these two specific issues as this is the final opportunity to put vital guarantees and protections into legislation,” the letter says.
However, No 10 has repeatedly underlined the prime minister’s determination that the bill be passed unamended.
It insisted no firm decision had yet been taken on a date for article 50, and the government remained committed to starting exit talks with the other 27 EU countries by the end of March.
But two government sources told the Guardian that ministers have been told Tuesday or Wednesday next week are possible, if the legislation passes smoothly. Otherwise, a series of events, including Dutch elections and the celebration of the 60th anniversary of the Treaty of Rome, could mean delaying until the end of the month.
If MPs are satisfied with what Davis has to say on Monday, Labour sources said that peers could concede, allowing the bill to pass, perhaps by 10pm that evening.
The prime minister is due to give a statement to the Commons on Tuesday reporting back on this week’s summit of EU leaders in Brussels. She could take that opportunity to announce she is invoking article 50.
May would also send a letter to the president of the European council, Donald Tusk, setting out Britain’s priorities in the negotiations. It is likely to reflect closely the plan set out in her Lancaster House speech in January, in which she said Britain would leave the single market and most aspects of the customs union, but would seek a comprehensive free trade agreement with the EU aimed at securing tariff-free access to its markets, and “frictionless” trade across borders.
Brussels will be keen to see how May and Davis will handle demands for up to €60bn (£50bn) in a “divorce bill”, to settle Britain’s liabilities for ongoing EU costs such as pensions.
Tusk has suggested the other 27 member states could press ahead rapidly with discussions about how to respond to Britain’s negotiating demands, and how the talks process should proceed. EU leaders expect to hold a summit in early April.
The Financial Times reported that EU officials in Brussels had been told to expect article 50 to be triggered imminently.