Theresa May will gift the Yes campaign a head start in any second independence referendum after signalling a move to let EU migrants keep the vote after Brexit.

In a key concession, the Prime Minister is understood to be ready to keep the existing Holyrood franchise in return for the same deal for British citizens on the continent.

Her move, widely flagged as she prepares to reboot talks with Brussels in a keynote speech in Florence next week, is expected to have "significant" repercussions in Scotland.

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Last month international experts predicted that EU migrants could swing a second referendum behind independence - provided Scots remained narrowly split on the issue.

However, Mrs May will set aside such forecasts as she seeks to find common ground with EU negotiators on citizens' rights and finance so the two sides can move on to meatier talks on trade, sources said.

The European Union - both its Commission and Parliament - have made it clear they want to see progress on a divorce settlement, including the rights of migrants and Britain's share of EU debts, before discussing a new post-Brexit relationship.

There is widespread speculation that Mrs May will use her Florence speech to pitch a 'transitional' deal first mooted by her chancellor, Philip Hammond, to keep paying in to the EU after formal Brexit in 2019.

A likely deal on voting rights was spelled out in unnoticed remarks in the House of Lords last week. Brexit Minister Baroness Anelay of St Johns, referring to talks so far this summer, said: "The EU does not plan to maintain the existing voting rights for UK nationals living in the EU.

"We have made it clear that we stand ready to protect the rights of EU nationals living in the UK to stand and vote in municipal elections."

This offer comes amid horse-trading over other rights, such healthcare and family re-union for millions of migrants, both EU citizens in the UK and 1.2m Britons who live on the continent.

Right now migrants have the right to vote in municipal elections and those for the devolved assemblies but not general elections, or last year's Brexit referendum.

They were able to take part in Scotland's 2014 vote. Foreign nationals eligible to vote, including EU ones, were slightly less likely to support independence in 2014 than the national average, with just under 43 per cent against and just over 57 per cent for.

Last month the Scottish Centre on European Relations (SCER) published an expert report suggesting that reversing those numbers would close the gap between the two sides, as of 2014, by two percentage points.

Some experts believe the migrant vote could swing behind Yes on the kind of scale usually only seen in rigged elections in dictatorships.

However, there is no polling to back this speculation up and there may well be migrant groups who either carry anti-separatist sentiments from their home countries - such as Spain or Romania - or who feel they have moved to Britain and want to remain in the UK.

SCER director and Brussels think tank veteran Kirsty Hughes, who commission last month's research, said keeping the current franchise may appear a minor concession amid complex negotiations.

But she added: "For Scottish politics it could prove highly significant."

There are around 180,000 EU citizens in Scotland, including children, just short of the number needed to overturn the 400,000 advantage for No recorded three years ago.

Polls would need to narrow for EU nationals to swing an independence referendum. The two most recent surveys put support for independence at 46 per cent, basically unchanged from the 45 per cent recorded at the actual vote in 2014. This includes a Survation poll for the Daily Mail put in to the field this week.

SNP sources are also delighted at the prospect of EU nationals keeping the vote in Holyrood elections, not least for some marginal constituencies, in Edinburgh, Moray or Perthshire with high migrant numbers.

Alyn Smith, the SNP MEP, said: "Our definition of Scottish is everyone who lives in Scotland, we proved that in the independence referendum, we celebrate the contribution New Scots have made to our society and economy.

"We want to see all EU nationals feel safe and welcome in Scotland, and having a vote is an important part of that. This welcome report shows that this is possible in whatever the future holds, I hope this disastrous UK government will give it proper attention."