European Union (EU) migrants arriving in the UK are expected to lose the right to stay permanently next month when the prime minister formally invokes Article 50, triggering the Brexit process.

Making the announcement, Theresa May is likely to say that those arriving before the cut off date will have their right to free movement assured as long as reciprocal rights are handed to Brits living in other member states.

But those arriving afterwards will be subject to rules drawn up during the negotiation of the terms of Britain’s exit to the bloc.

Calls have been made to set the cut-off date retrospectively to come into force on the day the British people voted in a referendum to leave the EU, while others have suggested the new rules shouldn’t come into force until Britain has formally left the EU (in two years’ time).

But a Whitehall source told the Daily Mail there were problems with both options: “Setting a retrospective date is fraught with legal problems and if you set a date two years into the future, you are giving people who might be considering coming here an awful lot of notice.

“So it makes a lot of sense to do it at or around the point when we trigger Article 50 next month.”

A government source concurred, voicing concern that setting a deadline two years hence would be a green light to EU migrants thinking of making the move. “We have had some suggestion that that the European Commission might attempt to force us to protect everyone who arrives up to the moment of departure,” the source told The Telegraph. “We could end up with half of Romania and Bulgaria coming here if we wait that long.”

Thrashing out the terms upon which EU citizens can migrate to and from a post-Brexit Britain is likely to be a key plank of the negotiations when they get underway, prompting the government to face down opposition party calls – most notably within the Lords – to release the details of their hoped-for terms before heading into the negotiating chamber.

Former minister Iain Duncan Smith said: “Theresa understands that if you want to take control you have to command the high ground. She will be giving clarity by setting a clear deadline while the European Union looks increasingly muddled and mean-spirited.”