A Hadrian’s Wall-style border might have to be built to stop illegal migrants flooding into England if Scotland gets independence.
Other dramatic possible consequences would be David Cameron having to give up part of his £3 billion EU rebate, and the UK’s voting strength in Brussels being slashed.
The potential side-effects of Scotland breaking away are outlined in a Foreign Office memo leaked to The Mail on Sunday.
The document refers to the remnants of the UK, were Scotland to go it alone, as the Orwellian-sounding ‘remaining-UK’.
The phrase has echoes of ‘the former Yugoslavia’ name given to the elements of the ex-Balkan state, such as Serbia and Kosovo, which became independent after a bloody civil war.
The briefing contradicts claims by Alex Salmond, Scotland's First Minister and Scottish National Party leader, that after independence the border could stay completely open.
The Foreign Office experts raise fears that a fully-fledged international border would be necessary because Scotland could be required to join the ‘Schengen’ open-borders system used by 25 European states which Britain has opted out of.
That would allow anyone to travel from continental Europe to Scotland without any internal border controls, meaning immigration and passport checks would be needed along the English/ Scottish border.
It would also mean Scotland would be ejected from the current ‘Common Travel Area’ comprising the UK, Ireland, the Channel Islands and the Isle of Man.
The report says: ‘Article 8 of the Schengen Protocol requires all new EU member states to accept the Schengen “acquis” in full.
A strict application of this provision in negotiations with Scotland .  .  . would make it practically impossible for Scotland to remain part of the Common Travel Area.
‘Scotland would not, by virtue of its participation in Schengen, be able to maintain its own border controls on people entering Scotland from the Schengen area.’ The memo also states:
Scotland would be forced to join the euro if it is regarded as a ‘new’ member by Brussels.
The UK would lose influence in Brussels as its voting strength would diminish.
Mr Cameron could be forced to surrender part of the UK’s £3 billion EU rebate to Scotland.
The ‘remaining-UK’ may itself be forced to re-apply to join the EU.
The memo flies in the face of repeated claims by the Coalition that Scotland’s economy will suffer a hammer-blow if it cuts links with the rest of the UK.
It states: ‘Whether the economic effects for Scotland of independent EU membership would be beneficial or adverse, compared with the present position, cannot be determined without knowing the terms on which independent membership was agreed.’

The Foreign Office document is believed to date from 2009, under the last Labour Government.
However, its core arguments will still be used by the Coalition in opposition to Scottish independence before the referendum, which is expected in 2014.
The memo warns that the process of Scotland ‘re-joining’ the EU could take up to three years – potentially depriving the country of vital Brussels’ aid for the whole period.
It says: ‘EU law would require negotiation of the terms of independent Scotland’s membership of the EU since the treaties do not provide for an increase in the number of member states other than by treaty amendment.’
London could raise objections if the split were not ‘amicable’, the document warns. ‘Remaining-UK would have a veto and could decide to make life difficult,’ it says.
Scotland’s fate would also hang on the goodwill of ‘other member states – some of whom, with regional problems of their own, such as Spain, Belgium and Italy, might not be anxious to demonstrate that independence yielded EU benefits’.
The report warns that once back inside the EU, Scotland could apply for a slice of the UK’s Brussels budget rebate if the newly-independent state finds it is a ‘net contributor’ to EU funds.
Last night, a spokesman for Mr Salmond dismissed the memo’s contents as ‘tired old scare stories’ which were untrue and insulting to Scotland.
He said: ‘As legal, constitutional and European experts have confirmed, the reality is that Scotland is part of the territory of the EU and the people of Scotland are citizens of the EU.
‘There is no provision for either of these circumstances to change upon independence, and the rest of the UK will be exactly the same position. The border will be the same. There’ll be no barbed wire or checkpoints.’
Hadrian’s Wall was built along the border by the Romans from 122AD.