A key group of EU states is threatening to totally sabotage Brexit

A bloc of EU member states in eastern Europe is threatening to veto any Brexit deal that would risk the right of their citizens to live and work in the United Kingdom.

In yet another reminder of how tough negotiations will be for Theresa May, Slovakian prime minister Robert Fico said on Saturday that his country, Hungary, Poland, and the Czech Republic would unite to block any deal that threatens of the free movement of people.

"Unless we feel a guarantee that these people [living and working in Britain] are equal, we will veto any agreement between the EU and Britain," Fico told Reuters.

"I think Britain knows this is an issue for us where there's no room for compromise."

One of the main reasons the majority of Brits voted for Brexit in June was the high level of EU citizens who have migrated to the UK in recent years. Leave campaigners like former UK Independence Party leader Nigel Farage repeatedly used phrases like "take back control" when talking about the nation's borders.

May and foreign secretary Boris Johnson have previously suggested they would pursue a Brexit deal which would allow Britain to retain access to the single market but opt-out the free movement of people.

However, speaking at a meeting of EU leaders on Saturday, Fico made clear that May would face an almighty challenge trying to strike this sort of deal. "The V4 countries will be uncompromising," he said. The "V4" is a term often used to describe the group of Slovakia, Hungary, Poland, and the Czech Republic.

Fico added that he and fellow European leaders would not let their citizens become "second class citizens" as a result of Britain's eventual withdrawal from the 28-nation bloc.

The Slovakian leader's warning comes just days after the EU Parliament controversially appointed Guy Verhofstadt to be its main negotiator in Brexit talks. The former Belgian prime minister was fully opposed to Brexit long before the referendum and shares Fico's opinion that Britain will not be allowed any access to the single market if it refuses to accept the free movement of people.

Farage described the appointment as a declaration of "war" on Brexit negotiations. The former UKIP leader, like other pro-Leavers, are worried that Verhofstadt's firm negotiating position will mean delivering Brexit will be near-impossible for May and the rest of her government.

The reality is that Fico and Verhofstadt's stance is no different to the rest of the Union. Jean-Claude Juncker, the EU President, used the meeting on Saturday to reiterate that Britain can forget single market access if it refuses to accept the free movement of people to Britain.

"There is a clear interlink as we made clear at the very beginning between the access to the internal market and the basic principles of the internal market - namely the free movement of workers and we are sticking to that position," he said.

"This is not a game between prime ministers leaving and prime ministers remaining, this is about people in Europe."