MEPs will block any Brexit deal unless EU citizens who move to the UK up to 2019 are granted the right to stay and work, the European Parliament's chief negotiator has said.

Guy Verhofstadt vowed to fight any attempt by Britain to set tomorrow – the day for triggering Article 50 – as the “cut-off date” for the free movement of people.

Both the Parliament and the Commission’s chief Brexit negotiator, Michel Barnier, are concerned by reports that No 10 will designate 29 March 2017 as the landmark date.

If so, EU nationals moving to the UK for the next two years, until Brexit is completed, would have weaker rights than those already in the UK – even if the rights of existing citizens are eventually guaranteed.

A five-page resolution setting out the Parliament’s red lines – to be voted on next Wednesday – will rule out any “degradation” of the rights of EU nationals arriving in the UK over the next two years.

Speaking to The Independent, Mr Verhofstadt warned Theresa May that imposing the cut-off date would be the wrong way to start the negotiations, which will begin after tomorrow.

“Any unilateral decision to curtail the rights of EU citizens in the UK, while the UK remains a member of the EU, would be contrary to EU law and we would oppose such a move vigorously.”

The Prime Minister has said the future rights of the three million EU nationals in the UK – and of around one million British ex-pats living in the EU – will only be settled on a reciprocal basis, as part of the Brexit negotiations.

EU citizens in UK victims of 'political games', says Guy Verhofstadt

However, government sources have briefed previously that setting 2019 as the cut-off date for any agreement would spark a rush of EU citizens seeking to enter the country before then.

Now the European Parliament’s resolution will insist on “equity, reciprocity, symmetry and non-discrimination” for all EU nationals – for as long as Britain remains a member state.

MEPs have the right to veto any deal between the EU and the UK when talks come to a close – and Mr Verhofstadt has already threatened to exercise that right, if necessary.

Gianni Pittell, the leader of the socialist bloc in the European Parliament, also confirmed that MEPs would not accept discrimination between EU citizens.

“We have heard that Theresa May is considering a cut-off date as the notification date,” he told The Guardian newspaper.

“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.”

The warnings come just hours before Sir Tim Barrow, the UK’s permanent representative to the EU, delivers the historic letter from Ms May to start the exit process.

At 12.30pm, the clock will start ticking on the two years of talks allowed under Article 50 of the Treaty of Lisbon.

The resolution is also expected to insist that Britain’s withdrawal agreement be enforced by the European Court of Justice – which is certain to inflame hardline Brexit supporters.