A group of Eastern European countries will veto any Brexit deal that diminishes the rights of their citizens who live and work in Britain, Slovakia's prime minister has said.

Robert Fico said the Visegrad Four (V4) - the Czech Republic, Hungary, Poland and Slovakia - want a guarantee that their nationals “are equal” before agreeing to any deal ahead of Britain leaving the European Union.

Prime Minister Theresa May has so far refused to guarantee the status of EU nationals in the UK, but insisted she wants them to stay after Brexit - if the rights of Britons overseas are respected.

Mr Fico told Reuters: “V4 countries will be uncompromising.

“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.

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

It comes after Downing Street poured cold water on claims that Ms May told one of Brussels' most senior figures that she wants to trigger the formal process to pull Britain out of the EU early next year.

European Council president Donald Tusk said the Prime Minister told him during talks at Number 10 last week it was “quite likely” she would be ready to trigger Article 50 of the Lisbon Treaty “maybe in January, maybe in February” 2017.

But a Downing Street source said Ms May did not specifically mention January or February at the meeting and that Mr Tusk's comments were an “interpretation” of their conversation.

The PM “recognises the need to deliver on the public verdict without delay”, the source added.

Formal negotiations between the UK and the EU cannot begin until she starts the two-year process, which Brexit Secretary David Davis has insisted will be triggered without a parliamentary vote.

Meanwhile, Defence Secretary Sir Michael Fallon vowed to block any proposals for an EU army while Britain remains a member of the union, in a move likely to anger European leaders.

In his state of the union address on Wednesday, European Commission president Jean-Claude Juncker called for EU countries “to pool their defence capabilities in the form of a permanent structured co-operation”, and proposed a European Defence Fund by the end of the year.

But Sir Michael said the UK would veto plans for any EU army that may rival Nato, as long as the country remained a member of the union.

“That is not going to happen,” he told The Times. “We are full members of the EU and we will go on resisting any attempt to set up a rival to Nato.

”We have always been concerned about unnecessarily duplicating what we already have in Nato.“

But former Liberal Democrat leader Lord (Menzies) Campbell said there was nothing the UK can do after Brexit to protect Nato from the potentially damaging effect of an EU army because it will not be able to veto its creation from outside the union.

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The peer, who is a member of the UK parliamentary delegation to the Nato Assembly, said: ”Even as a fervent European, I regard the creation of a European army as a deeply damaging, long-term threat to Nato.

“The cornerstone of European defence is Nato, of which the United States is the most senior partner, contributing 75% of the budget of the alliance.

”The creation of a European army will only encourage isolationists in the United States to argue that Europe should be responsible for its own defence.

“At a time when few of the Nato countries can meet the minimum requirement of 2 per cent of GDP defence expenditure, parallel headquarters and staff make no sense whatsoever. ”

