The British government is facing more opposition to its plans for leaving the European Union from Parliament's unelected House of Lords.

The Lords are likely Tuesday to pass an amendment to the government's Brexit bill requiring Parliament — not just the government — to approve Britain's exit deal with the EU. The chamber has already voted to promise EU citizens living in Britain that they can stay after the U.K. leaves the bloc.

But the changes may be temporary. Once the bill is passed by the Lords it will go back to the elected House of Commons, where lawmakers may overturn the amendments next week.

Prime Minister Theresa May wants to invoke Article 50 of the EU's key treaty, triggering exit negotiations, by March 31. She can't do that until Parliament passes legislation sanctioning the move.