Brexit minister David Davis has said there would be no sudden drop in numbers, as it would take years to fill low-skilled jobs in hospitality, social care and agriculture now done by immigrants.
After almost a year of phoney war since the June 23 referendum vote to quit, British negotiators led by Brexit Secretary David Davis will sit down with the European Union, possibly still in May.
U.K. Brexit Secretary David Davis has said "the first meeting, bluntly, will be about how we do this?"
The UK government is seeking "a deal that works for every nation and region of the UK and indeed for all of Europe - a new, positive partnership between the UK and our friends and allies in the European Union", Davis added.
"Everything is ready on this side", commission spokesman Margaritis Schinas said.
May 4 (tentative) - Tusk needs about four weeks to prepare a summit he will chair of the 27 to agree final guidelines and mandate the European Union executive's Michel Barnier to negotiate.
Due to trigger Article 50, her office has said May will be visiting Wales, Scotland and Northern Ireland to hear the government's differing views on Brexit.
"The phony period is almost over, and the real work of negotiations are about to begin", McFadden said.
"This Government's Plan for Britain will seek to deliver a stronger, fairer United Kingdom and a better deal for ordinary working people in Wales and across the nation".
Scottish Government minister Michael Russell complained that the UK Government "somehow forgot to inform" the devolved administrations of the timetable for Article 50, leaving them to hear about it from the media.
Once that is received, there will be a two-year negotiating process before the United Kingdom finally breaks away in 2019.
It's also not clear what rights the estimated 3 million European Union citizens already working and living in Britain will retain. May has said now is not the time for Scotland to vote on independence, preferring to wait until after Brexit.
The BBC's Ben Wright said he expected the Article 50 letter to be short, possibly extending to two pages at most, and for Mrs May to use it to publicly reiterate her general objectives - such as leaving the single market but reaching a mutually beneficial agreement on trade and other issues.
We assumed that there would be a deal on preserving the existing rights for EU living in Britain, and British citizens living in the EU and that Britain and the EU would continue to co-operate on security matters.
The European Parliament also wants a say, and will have to approve the final deal between Britain and the bloc. For those who would like Britain to stay after all, the 27% unsure of what they would favour in those circumstances does hint that opinion might change if negotiations go badly enough.