LONDON (AP) — The British government promised Tuesday that it won't strip European Union nationals of any rights without lawmakers' approval, as it tries to persuade Parliament to authorize the start of divorce talks with the bloc.

By leaving the EU, Britain will be withdrawing from the bloc's policy of free movement of citizens among member states. That leaves 3 million EU nationals in Britain, and 1 million Britons living in other member countries, in limbo.

Parliament's upper chamber, the House of Lords, plans to try Wednesday to pass a guarantee that EU citizens would be able to stay in Britain after Brexit.

Opposition peers hope to amend a bill authorizing the start of EU exit talks to include the promise, and have enlisted the support of some parliamentarians from the governing Conservatives.

In an attempt to prevent the authorization bill's defeat, Home Secretary Amber Rudd wrote to members of the House of Lords saying "nothing will change for any EU citizen, whether already resident in the U.K. or moving from the EU, without Parliament's approval."

But the letter offers no guarantees of EU citizens' right to remain, saying only that the issue would be a top priority once formal exit talks start.

Prime Minister Theresa May plans to trigger Article 50 of the EU's key treaty, starting two years of exit negotiations, by March 31.

But she can't do that until Parliament passes legislation sanctioning the move. The House of Commons approved the bill earlier this month and the Lords is scrutinizing it this week.

If Lords amend the bill, it will have to go back to the House of Commons for another vote, delaying its passage and potentially threatening May's timetable.

A growing number of politicians and business groups are warning that the schedule already is tight. Former Prime Minister John Major warned Monday that the goal of agreeing on divorce terms within two years is "very, very optimistic."

The British Chambers of Commerce said Tuesday that the break from the EU should be delayed if a post-Brexit trade deal remains incomplete after two years. The business group said Britain must avoid "sudden disruption to our trading relations" with the EU and should extend the negotiating period if necessary.

Foreign Secretary Boris Johnson told the group's conference that he wanted to go "full tilt and get it done within two years."

"I'm not saying there won't be some bumps in the road," Johnson added. But he said the U.K. and the bloc should be able to strike "a fantastic deal" that is good for British businesses.