IT is the beginning of the end of Britain’s membership of the EU today, with Theresa May triggering Article 50, the formal process for leaving Brexit.

While the Prime Minister chairs a meeting of the Cabinet this morning, the British Ambassador to the EU, Sir Tim Barrow will hand a letter, signed by May, to European Council president Donald Tusk.

In a highly co-ordinated move it should be in the hands of the EU chief by around 12.30pm UK time, just as the Prime Minister will stand up in the House of Commons to make a statement to MPs confirming the two-year countdown to Britain’s departure from the EU is finally under way.

However, even before talks have started, there is an argument brewing over the rights of EU citizens who arrive in Britain from today. The British Government is understood to want today to be the “cut-off date” for the free movement of people.

Any EU nationals moving to the UK from today, should, the Government believe, have different rights from those already in the country.

The European Parliament, however, say there should be no change to the status of any EU national until the Brexit process finishes in 2019.

According to The Guardian a five-page resolution outlining the European Parliament’s red lines, to be voted on by MEPs next week, will insist on “equity, reciprocity, symmetry and non-discrimination” for all EU nationals while Britain remains a member state.

Leader of the socialist bloc in the European Parliament, Gianni Pittella, told the paper: “We have heard that Theresa May is considering a cut-off date as the notification date. We completely disagree on this and we believe that the British citizens and those from the other 27 states are EU citizens until the day of the divorce.

“During this period the UK is a member state with full rights and obligations.

“It cannot be right that someone signing a work contract in the UK on Tuesday has more rights than someone signing a contract on Thursday.”

Meanwhile, a leaked letter from David Davis to SDLP MP Mark Durkan, seemed to suggest Northern Ireland would be able to join the European Union as part of the Republic of Ireland after Brexit, if it votes for reunification.

According to The Times, the Brexit Secretary, wrote: “If a majority of the people of Northern Ireland were ever to vote to become part of a united Ireland the UK Government will honour its commitment to enable that to happen.

“In that event, Northern Ireland would be in a position of becoming part of an existing EU member state, rather than seeking to join the EU as a new independent state.”

Davis told Durkan that the Government’s “clear position is to support Northern Ireland’s current constitutional status: as part of the UK, but with strong links to Ireland”.

Ministers and government lawyers say the situation is similar to Germany before reunification. When the GDR reunited with West Germany in 1990, it automatically became part of the European Community.

In that scenario, Northern Ireland would have to adopt the euro but, unlike Scotland, would not have to apply for membership in its own right. Under the terms of the Good Friday Agreement, Dublin and London must agree to a vote on Irish reunification if there is demand.

There was some succour for Davis and the Prime Minister amongst all this though. A YouGov poll suggested voters want the Government to push ahead with Brexit, even if it means no deal and taking the country off a cliff edge into the tariffs and difficulties of World Trade Organization rules.

Failing to secure a trade agreement seems increasingly possible. On Monday, Davis said while “no deal is not as easy as some would have you believe” it would be “a lot better” than critics had claimed.