Joining EU 'difficult if not impossible' for Scotland, Barroso warns

"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 other [existing member states]," said Barroso.

"We have seen that Spain has been opposing even the recognition of Kosovo, for instance, so it's to some extent a similar case because it's a new country," he added.

The Scottish government says that if voters choose independence in the September referendum then it would re-write and agree the country's revised membership terms in the eighteen months after the vote.

During this time Scotland would still be officially part of the UK before becoming independent in 2016.

It also believes that the country would be able to join the EU by using Article 48 of the EU treaty which would avoid the need for a formal accession process.

But Barroso appeared to dash these hopes as well.

"In case there is a new country, a new state, coming out of a current member state it will have to apply," he said. "Accession to the European Union will have to be approved by all other member states of the European Union," he added.

Barroso's remarks were quickly dismissed as "preposterous" by Scotland's deputy first minister, Nicola Sturgeon.

"This is a preposterous assertion – as the ridiculous comparison with Kosovo illustrates. Scotland is already in the EU and has been for 40 years," she said.

"The fact of the matter is that no member state has said that it would veto Scotland's continuing membership."

However, Barroso's statements will still come as a blow to the Scottish nationalist government, which believes that it would face few problems in negotiating its terms as a new EU member state.

Scots will vote on 16 September to decide whether or not to become independent from the UK.

Two months later, voters in Catalonia could also have their say on whether to secede from Spain, although the Spanish government is planning to prevent a referendum which it claims would be illegal under the country's constitution.