The House of Lords is likely to oppose the legislation that gives Prime Minister Theresa May the power to begin the process of Brexit, according to a former Labour Party minister.

The House of Commons approved the legislation earlier this month and the lower chamber is due to start debating the bill on Monday.

While the legislation passed through the Commons, where May’s party has a majority, it could face a harder time through the Lords, where the Conservative Party constitutes the minority.

Peter Mandelson, a Labour member of the House of Lords, said in an interview with the BBC that the government could be defeated on two issues in particular: the amendment on post-Brexit rights for EU citizens living in Britain and the one calling for parliament to have a vote on the final deal.

“I think there is a strong body of opinion, across party and amongst independent peers as well that both these issues are very serious,” he said.

If the Lords succeed in amending the existing legislation, the bill will have to go back to the lower house for approval and May might not be able to trigger Article 50 of the EU’s Lisbon Treaty by the end of March as she planned.

The legislation could potentially be passed back and forth between the Lords and the Commons until they both agree.

“At the of end of the day the House of Commons must prevail because it is the elected chamber but I hope the House of Lords will not throw in the towel early,” Mandelson said.