Debate on the government’s Brexit bill is set to resume in the House of Lords.
Peers have been urged to “respect” voters’ decision to leave the EU but opposition parties say “reasonable changes” could be made to the bill.
MPs have already backed the proposed law, authorising Mrs May to inform the EU of the UK’s intention to leave.
There is unlikely to be a formal vote at the end of Tuesday’s Second Reading debate in which a record 190 peers are scheduled to speak.
Monday’s sitting was extended to midnight to allow more peers to take part and will resume at 11:00 GMT.
The government does not have a majority in the Lords and opposition and crossbench peers are seeking guarantees about the rights of EU citizens in Britain and the role of parliament in scrutinising the process.
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Mrs May has said she wants to invoke Article 50 of the 2009 Lisbon Treaty – the formal two-year mechanism by which a state must leave the EU – by the end of March, and the government has warned the House of Lords not to frustrate the process.
The prime minister watched Monday’s opening proceedings in person as leader of the House of Lords Lady Evans said the government had promised to deliver on the result of last year’s referendum, in which 51.9% of voters backed Brexit.
She said the bill was not an opportunity to “revisit” last year’s referendum debate and peers should “respect the primacy of the elected House and the decision of the British people”.
For Labour, Lords opposition leader Baroness Smith of Basildon said the government would not be given a “blank cheque” and that “if sovereignty is to mean anything, it has to mean parliamentary responsibility”.
And Lord Newby, leader of the Liberal Democrats in the Lords, said the bill could be changed and sent back to the House of Commons for reconsideration, arguing there was a “world of difference between blocking… and seeking to amend it”.
The government has set aside five days in total to discuss the various stages of the EU (Notification of Withdrawal) Bill – starting with its Second Reading, in which peers debate the general principles of the bill.
The Second Reading debate is due to conclude on Tuesday evening – possibly with a vote, but only if peers break with their usual practice of allowing government legislation through unopposed at this stage.
Although amendments are not voted on at this stage, speeches are being closely watched for signs of the mood of peers on the two key ones of parliament having a “final meaningful vote” on the draft Brexit agreement – and guaranteeing the rights of EU citizens in the UK.
Detailed scrutiny of the bill at committee stage is due to take place on 27 February and 1 March. If the bill is not amended, then it could theoretically be approved by the Lords at Third Reading on 7 March, becoming law shortly afterwards.
If peers do make changes to the bill, it would put them on a collision course with MPs – who overwhelmingly passed the bill unaltered and would be expected to overturn any Lords amendments.
Although the Conservatives have the largest number of peers in the Lords, with 252 members, they are vulnerable to being outvoted if opposition peers – including 202 Labour peers and 102 Lib Dems – join forces. Much will hinge of the actions of the 178 crossbenchers – who are not aligned to any party.