Any bid by an independent Scotland to gain full European Union membership is unlikely to be vetoed by existing member states, according to a constitutional expert.

Pro-union campaigners have claimed Scotland could be refused entry by countries hoping to discourage support for independence movements within their own countries.

Spain and Belgium both have significant internal independence movements in Catalonia and Flanders respectively.

Professor Michael Keating from Aberdeen University, who is also director of the Centre for Constitutional Change, has talked down the prospect of such a move in his new book, Debating Scotland: Issues of Independence and Union in the 2014 Referendum.

He said: "None of them [EU member states] threatened to veto Scotland's ascension to the EU and their representatives usually said that they would follow the UK's lead; the inference is that they would recognise an independent Scotland.

"In that case, it is difficult to see on what grounds they could veto Scotland's EU membership."

The Spanish government has never formally said it would veto a Scottish appliance but has warned of the negative affects of Scottish independence.

On the day before the 2014 referendum, Spanish prime minister Mariano Rajoy said Scottish independence would be "bad for the region, for the state and for the entire EU [because] they affect the wealth, employment and welfare of all the citizens, as well as the very essence of the EU".

Then-Spanish foreign minister Jose Manuel García-Margallo y Marfil said independence would be a "catastrophe"

Scottish Conservative constitution spokesman Adam Tomkins MSP said: "The fact is countries like Spain would have far more reason to veto an independent Scotland's membership than accept it.

"That's been made abundantly clear by European leaders on a number of occasions.

"If the SNP had its way we'd be out of the United Kingdom union, and out of the European Union.

"No-one's saying a separate Scotland would never be granted access to the EU.

"But it would have to do see on the EU's terms, and that could mean adopting the Euro, signing up to Schengen and not receiving any of the unique terms the UK had."

Entry into the EU must be ratified by all existing member states.

The UK's own entry to the the precursor organisation to the EU, the European Economic Community, was vetoed by the French government in 1961 and again in 1967.

Five states - Albania, Macedonia, Montenegro, Serbia and Turkey - have applied to join the EU but have not yet been admitted.

Then-prime minister David Cameron said if Scotland left the UK it would have to "queue up as it were behind other countries, for instance those in the Western Balkans, that are already on the path towards membership."

Prof Keating noted there is no such thing as queue for membership, however, and countries are assessed on their merits not when they applied.

Turkey first applied to join in 1987 before ten current EU member states even became independent countries.

"In practice, there is no queue and applicant countries are accepted when they are ready, which makes it much easier to for northern European countries that already meet that criteria," Prof Keating argues.

His comments come last month after a pair of academics said it was "not obvious" that Spain would veto a Scottish application.