The European Union (EU) is to demand that migrants who come to the UK for years after Brexit be allowed to stay permanently, a move which could disrupt negotiations.

Michel Barnier, the EU’s chief negotiator, has drawn up new negotiating guidelines, demanding that unlimited mass migration from the bloc should continue throughout the agreed two-year so-called ‘Brexit transition period’.

The negotiating directive, seen by The Times, urges the EU to resist the UK applying any new immigration controls after it leaves the bloc – EU citizens and their families, including non-Europeans, would be eligible for automatic residency rights for an indefinite period if they arrived in the UK after Brexit but before 2021.

Iain Duncan Smith, a former Tory cabinet minister and a Eurosceptic, told the paper: “We should smile and do nothing.”

Poland and other Central and Eastern European states reportedly pushed for the new stance, as they were concerned at Britain’s ability to restrict mass migration after March 2019.

According to EU officials and diplomats, the new draft states: “In particular, the provisions of the citizens’ rights part of the withdrawal agreement should apply from the end of the transition period.”

The demand seems to betray an agreement struck in December last year, that only migrants arriving up until the Brexit date in 2019 could stay. The UK had initially wanted the cut-off date to be the day of the Brexit vote in 2016.

“The British always tell us that nothing is agreed until everything is agreed. Here is an example of exactly how that works in practice,” one EU source said.

“The deal in December did specify March 2019 for free movement rights. That was then. Now as part of the discussion on transitional arrangements that has changed.”

In November last year, a European court ruled that EU citizens who become British should have more rights than other Britons to bring in spouses from outside the bloc to live in the UK.

The European Court of Justice (ECJ) rejected the claim that all British citizens should be subject equally to the Home Office’s stricter immigration regulations on gaining residency.

Europeans who gain British citizenship, for example, will not need to meet an earnings threshold of £18,600 a year to bring in a partner from outside the EU, unlike those born in the UK, the ECJ insisted.