Ed Miliband today issues the stark threat that manned border posts could be introduced if Scotland backs independence in next week's historic vote.

With just 11 days to go until the referendum - and polls showing growing momentum for Alex Salmond's nationalists - the Labour leader has urged voters to face up to the consequences of severing the 300-year-old Union.

'If you don't want borders, vote to stay in the United Kingdom,' Mr Miliband said in an interview with today's Scottish Mail on Sunday.

Scroll down for video

Passport please: Our artists' impression of a road border between England and Scotland post-independence

Asked whether that would mean him introducing border guards and passport checks if he was Prime Minister, Mr Miliband warned: 'It would have to be looked at.'

And Mr Miliband's spokesman added last night: 'The last time I looked there were two sides to the border - and we would be in charge of one of them. It would be up to us, not Mr Salmond, to secure our northern border.'

The prospect was raised as a shock poll showed that 51 per cent of Scots would vote ‘Yes’ to going it alone with just 49 per cent saying ‘No’.

The results of the YouGov survey means that the ‘Yes’ campaign has overturned a 22-point lead within a month.

Until now, Mr Salmond’s SNP has insisted that a separate Scotland would be part of a common, free movement area with the rest of the UK and Ireland.

Guards on the border: Where the controls could be placed if Scotland wins independence in 11 days' time

But a Downing Street source backed Mr Miliband, saying: ‘All the research shows that if there is a material difference between the immigration policy adopted by an independent Scotland and the rest of the UK, then border controls would have to be introduced.’

It raises the extraordinary prospect of drivers having to stop at manned barriers on the borders to show their passports.

SCOTS FOOTBALL LEGENDS JOIN CALLS FOR PATRIOTS TO SAY 'NO' Soccer pundit Alan Hansen and a ‘squad’ of great Scottish footballers from all eras of the game have called on ‘every patriotic Scot’ to vote No to independence. The other players who signed up to a statement are: Alex McLeish, Jim Leighton, David Moyes, Willie Miller, Paddy Crerand, Davie Provan, Barry Ferguson, Bertie Auld, Denis Law, Ally McCoist, Derek Johnstone, Murdo MacLeod, Ian Durrant, John Brown and Frank McAvennie. Advertisement

With the latest opinion polls showing Mr Miliband on course to win May’s General Election, a Labour Government could be in charge of negotiating Scotland’s passage to independence by March 2016 if the referendum returns a ‘Yes’ vote.

The SNP dismissed the idea of border controls as a ‘scare story’, but Mr Miliband’s advisers say it is not an idle threat: a secret memo drawn up by the Foreign Office in 2009 warned that a secure border would have to be established to protect the ‘remaining UK’ if an independent Scotland applied to join the EU.

All new EU members are obliged to accept the Schengen open borders scheme, meaning travellers from the continent would be free to fly into Scotland without immigration checks.

But Britain has opted out of the scheme, and erects border controls to vet travellers from Schengen countries, raising the prospect of a new Hadrian’s Wall between the two nations.

New border posts would cause chaos for travellers and prove to be hugely damaging to business.

The 95-mile border, which runs from Lamberton on the east coast to the Solway Firth in the west, cuts through 21 roads, including two major trunk routes, the A1 and M74.

No idle threat by Mr Miliband: A memo drawn up by the Foreign Office in 2009 warned that a secure border would have to be established to protect the ‘remaining UK’ if an independent Scotland applied to join the EU

A total of 23 million vehicle crossings a year are made on the roads, in addition to seven million passenger journeys on the West Coast and East Coast railway lines.

If controls were set up, immigration staff could man barriers on the roads while the train terminals would have to install checks similar to those adopted by Eurostar.

Last night former Labour Home Secretary David Blunkett echoed Mr Miliband’s warning by referring to last week’s riots in Calais when immigrants attempted to storm a ferry.

‘Were the Scots to vote “Yes” to separation, the issue of border controls and the threat of entry through Scottish ports and airports would take on serious proportions,’ Mr Blunkett told The Mail on Sunday.

Support for Alex Salmond's camp has soared, with a poll last night putting the Yes vote on 51 per cent

‘This is one issue that has not yet emerged as a key part of the debate over Scotland’s future. There is much at stake and little sign of Government action in response.’

Panic spread through the ‘No’ campaign last week after an opinion poll showed that its lead over the nationalists had shrunk to just six points, with support for Mr Salmond’s position up by eight points in a month.

Mr Cameron is happy to let Mr Miliband make the running for the pro-Union campaign as Labour supporters make up a large proportion of the ‘undecided’ voters.

The Prime Minister paid his annual visit to the Queen at Balmoral yesterday, but accepted the advice of Palace aides not to join her at the Braemar Games in case it was seen as ‘politicising’ the Monarch.

It is also in Mr Miliband’s interest to defeat the nationalists: if Scottish voters had been excluded from the last Election, Mr Cameron would have won a clear Westminster majority without the need for a Coalition.

The scale of Labour fears over a possible ‘Yes’ vote was laid bare yesterday when the party’s English and Welsh MPs were told to get up to Scotland to save the Union. In an email to all Labour MPs, Chief Whip Rosie Winterton revealed that Labour had booked scores of train tickets to get them to go north of the border before the referendum.

In his interview, Mr Miliband also mentioned his ‘personal connection’ with Scotland – his father, Ralph, was stationed at Inverkeithing, Fife, during the Second World War. ‘The rest of the UK would be much weaker without Scotland. It matters for reasons of heart and for head,’ he said.

Former Liberal Democrat leader Charles Kennedy urged fellow ‘No’ campaigners to hold their nerve, saying: ‘There is a flavour of detail and substance being swept aside in a tide of emotion and exclusive patriotism. It is a dangerous time for the No campaign.’

Asked about Mr Miliband’s remarks, a spokesman for Yes Scotland said: ‘There will be no border posts between an independent Scotland and the rest of the UK.

‘The No campaign will say anything to try to scare people into throwing away this unique opportunity to put our future into our own hands. They’ll always put what’s right for Westminster above what’s best for Scotland. Border posts manned by guards take the prize for one of the sillier scare stories.’