THE SNP's claim that an independent Scotland would join the EU seamlessly has been dealt a major blow after the Prime Minister of Spain warned that any new state would be "left out" and required to apply for membership.

Mariano Rajoy told a press ­conference in Madrid yesterday that the "consequences of secession" should be presented in a realistic way to Scottish voters.

He spoke out a day after First ­Minister Alex Salmond claimed an independent Scotland would be fast-tracked into the EU.

The Scottish Government yesterday published a detailed document expanding on its case for bypassing the usual, often protracted, EU accession procedures.

But asked about Scotland during a joint news conference with French President Francois Hollande, Mr Rajoy said: "I would like to see the consequences of secession presented to Scots in a realistic way. Citizens have the right to be well informed.

"If part of a Member State becomes independent, it would be left out of the European Union, and it would be good for citizens (of the EU) and Scots to know that. EU entry would need to be approved by all 28 Member States."

In an apparent message to ­seccessionist movements in his own country, he added: "Unity is not contrary to autonomy but a strength."

Last year the President of the ­European Commission, Jose Manuel Barroso, indicated that any new independent country would have to apply to join the EU.

However, Prime Minister Rajoy's comments are the clearest yet from the leader of an EU member state, with a formal say on the accession procedure, that an independent Scotland's membership may not go as smoothly as the SNP have claimed.

He spoke out as the Scottish ­Government published a new paper, based on advice from the Lord Advocate, arguing an independent Scotland could avoid the usual membership procedure conducted under Article 49 of the Treaty on the European Union.

Instead it said Article 48 allowed for a simplified process.

A UK Government spokesman described the plan as "a very novel interpretation of EU law" but SNP ministers Fiona Hyslop and Humza Yousaf have this week embarked on a charm offensive to promote the Scottish Government's proposed framework, holding meetings for European diplomats in Edinburgh and London.

Alistair Darling, the head of the pro-UK Better Together campaign said: "This is another blow to Alex Salmond's claims that nothing would change if we vote to go it alone. The Spanish Prime Minister has just made it clear that everything would change.

"We now know what the position of the Spanish government would be if we vote for independence. This has created even more uncertainty."

Scottish Liberal Democrat leader Willie Rennie said: "The Nationalists insisted an independent Scotland would not be put outside the European Union but this intervention from the Spanish Prime Minister directly contradicts this assertion.

"Hard won opt-outs on Schengen, the euro and the rebate would be in doubt and might have to be traded away to get back in."

Scots Tory leader Ruth Davidson said: "The Spanish Prime Minister has just blown Alex Salmond's case for EU entry out of the water. We need to know what advice the SNP received before they laid out their threadbare case in the White Paper, whether they'd spoken to other member states or even checked basic facts with EU officials."

Writing in the foreword to the ­Scottish Government's new document, entitled Scotland in the European Union, Deputy First Minister Nicola Sturgeon said: "The Scottish Government recognises that membership of the EU will require negotiations with other Member States and the EU institutions in order to agree the terms on which an independent Scotland will become a full member of the EU.

"We also believe - and this has been echoed by independent experts - that these negotiations could be completed within the 18 month period between a Yes vote in September next year and day one of independence in March 2016, ahead of the May parliamentary elections.

A spokesperson for Ms Sturgeon said: "We note that the Spanish Prime ­Minister has said he has not read our proposals, but Mr Rajoy has previously indicated that he considers the Scottish and Catalan situations are 'absolutely and totally different'.

"That is because the process for ­Scotland becoming independent is enshrined in the Edinburgh Agreement, where the UK Government has pledged to respect the outcome of the referendum.

"In the papers we have published outline we have detailed a process which will see Scotland negotiate its position as an independent member of the European Union from within, during the 18-month period between a Yes vote and independence day - a period when we will still be part of the EU as part of the UK, and which has been described as 'realistic' by the UK Government's own legal adviser.

"That process, under Article 48 of the Treaty of the EU, allows for Scotland to become a member state at the point of independence."