Nationalists reacted with fury to a claim by the President of the European Commission that an independent Scotland would find it “extremely difficult, if not impossible” to join the European Union.

Jose Manuel Barroso said countries such as Spain – which faces its own secession demands from Catalan and Basque separatists – could veto its entry.

But senior Scottish National Party figures described his claims as “preposterous” and “nonsense”.

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It is the second major policy setback in a week for First Minister Alex Salmond, who has angrily accused Westminster of “bullying” and “anti-democratic” tactics after all three main parties ruled out a go-it-alone Scotland being able to carry on using the pound.

Senior figures within the Yes campaign are now reportedly considering a plan B, which would see a fully independent Holyrood existing outside full currency union with the remaining parts of the UK.

Mr Barroso said he did not seek to intervene in the internal democratic process, but compared Scotland’s ambition to the plight of Kosovo, once part of Yugoslavia, which has had its membership opposed by Spain, Cyprus, Greece, Romania and Slovakia. He said that all member states would be required to support Scotland’s accession.

Mr Barroso told BBC1’s Andrew Marr Show: “Of course it will be extremely difficult to get the approval of all the other member states.”

He added: “We have seen Spain has been opposing even the recognition of Kosovo, for instance. So it is to some extent a similar case ... and so I believe it’s going to be extremely difficult, if not impossible – a new member state coming out of one of our countries – getting the agreement of the others.”

But Scotland’s Deputy First Minister Nicola Sturgeon said there was no evidence that other countries, including Spain, would veto an independent Scotland.