TONY Blair has raised the prospect that the historic vote for Britain to leave the European Union could at some point be reversed, saying the country should keep all its options open because the “will of the people” could change.

The former Labour prime minister argued that the UK Government should delay triggering Article 50, the two-year process for formally quitting the bloc, "for as long as it takes to get an idea of how the other side looks".

A downturn in economic fortunes, he suggested, could lead to a change in attitudes among voters about the country's future outside the EU.

Mr Blair told the Murnaghan programme on Sky News: "One of the reasons why we should keep our options open is that, yes, the referendum expressed the will of the people, but the will of the people is entitled to change.

"Right now, over the next two months, even while this psychodrama within the Conservative Party is going on, we've got to have the national interest protected by trying to set the scene for any negotiation."

He said he did not think the “settled will of the people” could be overriden but then pointed out the result was very close at 52 per cent to 48.

“Supposing some weeks or months down the line, as it becomes clear what we are moving to, as that becomes clear, if it becomes clear these terms are bad for us, if people start to worry about their jobs, we should just keep our options open.

"I'm not saying we should have another referendum; I'm not saying you can revisit this. I'm simply saying there's no rule about this; we're a sovereign people we can do what we want to do," added the former Labour leader.

Mr Blair’s views echo somewhat the arguments that have been made by some, most notably the Scottish Nationalists, about the Scottish independence referendum; that if the will of the people changed, then the subject would have to be revisited.

Meantime, concerns were raised about the position of those EU nationals living and working in the UK once Brexit happened.

First Minister Nicola Sturgeon has written to David Cameron as well as the five candidates in the Conservative leadership contest, calling for “immediate guarantees” for the rights of 173,000 EU citizens currently in Scotland.

In a separate development, a letter signed by prominent figures in both the Remain and Leave campaigns, including Gisela Stuart, who chaired Vote Leave, warned of “considerable anxiety for the three million EU citizens who have made their homes in the UK and the 1.2m British citizens living in other EU countries.”

They called on the Tory candidates to offer an “unequivocal statement” that EU migrants in the UK were welcome and that any changes post-Brexit would only apply to new migrants.

Theresa May, the frontrunner in the contest for the Conservative crown, stressed how Britain was still a member of the EU and no changes would happen while this was the case.

She explained: “But, of course, as part of the negotiation we will need to look at this question of people who are here in the UK from the EU.

“I want to ensure that we are able to not just guarantee the positions of those people but guarantee the positions of British citizens in other member states,” she added.

On the issue of when to trigger Article 50, the Home Secretary declined to set a deadline but stressed how the UK would need to establish its negotiating position before the clause in the Lisbon Treaty was triggered and all efforts had to be made to get the best deal for Britain on free movement and trade.

Her rival Michael Gove has said that Article 50 would not be triggered in this calendar year as the UK Government held preliminary consultations with its EU counterparts.

Both views contrast with those of other contenders. Leading Brexiter Andrea Leadsom, the energy minister, who launches her campaign on Monday, suggested the process of leaving the EU should start immediately. "We need to get on with it. We need to seize the opportunity," she insisted.

Meantime, fellow Leaver, Liam Fox, the former defence secretary, even pencilled in a date for Brexit: January 1 2019.

The views came as Nigel Farage, the Ukip leader, prepares to make his first keynote speech on Brexit on Monday in London, following the historic vote of June 23.