THERESA May has been urged by a former cabinet colleague to “flesh out” her plans on Brexit and warned not to give so-called hard Brexit supporters the space to dominate the EU debate.

Nicky Morgan, who was sacked as Education secretary by the Prime Minister, urged Mrs May to reveal more details about her Brexit strategy after a hardline Eurosceptic group was set up with the support of Tory former ministers.

Leave Means Leave has the backing of Tory former frontbenchers, including Owen Paterson, Dominic Raab and Sir Gerald Howarth.

Mrs May has been criticised for repeating the slogan "Brexit means Brexit" but failing to set out details of what she wants to gain from negotiations from the EU; the PM insists she will not offer a "running commentary" on her strategy.

Mrs Morgan told ITV's Peston on Sunday: "I do think that it's time to flesh out some of the issues, particularly around Brexit.

"You are seeing today that there are people in the Conservative parliamentary party now saying they are going to set up a sort of hard Brexit group.

"If you leave a vacuum other people will fill it and therefore I think the time is now to say - 'this is what we would like to get out of Brexit'."

Meantime, Cabinet minister David Lidington insisted the status of EU nationals living in Britain remained on the negotiating table despite a warning any Brexit deal which diminishes their rights could be vetoed.

Slovakian PM Robert Fico said the Visegrad Four [V4] - the Czech Republic, Hungary, Poland and Slovakia - want a guarantee their nationals "are equal" before agreeing to any deal ahead of the UK leaving the EU.

Mrs May has so far refused to guarantee the status of EU nationals but insisted she wants them to stay after Brexit if the rights of Britons overseas are respected.

Mr Lidington reiterated her position, telling BBC One's Sunday Politics: "Clearly, everything to do with EU citizens already here and prospective inward migration by EU citizens here, or British citizens to other countries, is part of the negotiation and that will be looked at in the round."

Mr Fico had 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.

"Britain knows this is an issue for us where there's no room for compromise," he added.

Amid growing calls for more details on the Government's Brexit strategy, Downing Street on Saturday poured cold water on claims Mrs May told one of Brussels' most senior figures that she wanted to trigger the formal process to pull Britain out of the EU early next year.

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

A Downing Street source said Mrs 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.

Documents leaked to The Sunday Times suggested International Trade Secretary Liam Fox and Foreign Secretary Boris Johnson were seeking a "hard Brexit" through a withdrawal from the EU customs union.

But Chancellor Philip Hammond is keen to remain in the bloc, which imposes tariffs on goods traded with countries outside it, according to the newspaper.

The legal advice prepared for Dr Fox also revealed that Britain could face fines from the European Commission for negotiating trade deals with countries outside the EU while the UK remains a member, the report said.

Labour seized on the remarks, insisting Dr Fox was being "cavalier" in his approach to trade talks.

Shadow international trade secretary Barry Gardiner said: "It is precisely this mixture of arrogance and incompetence by Liam Fox that will cause major problems for the UK in our Brexit relations with Europe.

"Fox seems to think he can just ignore the rules and that the EU will neither notice nor seek to retaliate in any way. Mr Fox has a record of acting as if the rules do not apply to him.

"The Prime Minister must realise that if she wants to protect the UK's interest and secure a new relationship with Europe that protects access to the single market and controls immigration then she must stop cavalier ministers like Fox from alienating the very people on whom our future relationship with Europe depends."

Elsewhere, the Government has been urged to publish its economic analysis of leaving the EU.

Brexit Secretary David Davis has revealed he is conducting a "quantitative" and "scientific" sector-by-sector analysis of the potential impact on the economy and potential policy solutions to harness the opportunities and mitigate the risks.

But he has also stressed that the UK Government is unlikely to reveal much about its negotiating strategy and plans in public before triggering Article 50; the formal process of leaving the EU.

Tory former attorney general Dominic Grieve, who supports the pro-single market Open Britain campaign, said: "The Government must make public all its internal analysis so Parliament and the country have a chance to scrutinise its decision-making.

"As they have the information, it should be made public in the interests of transparency and so people can make their own judgements," he added.