Confidential communciations reveal the British ambassador in Madrid has been confronted with mounting fears in Spain that the 2014 referendum in Scotland will spark demands for similar polls in Catalonia and the Basque Country.
The documents show UK ministers have been briefed to reassure their Spanish counterparts that Scotland's constitutional question is quite different from those of Spain's minority nations and regions.
Britain's ambassador in Madrid, Giles Paxman, in a telegram marked "restricted", has set out the UK's official response to such speculation.
He said: "We need to step up our efforts to explain why that debate is being played out in different circumstances and why the search for a legal, fair and decisive resolution to Scottish pressure for greater autonomy need not be a threat to Spanish territorial integrity."
Mr Paxman, whose elder brother Jeremy presents Newsnight, stressed the Spanish Government would be unlikely to allow a "regional" plebiscite along the lines of that planned by First Minister Alex Salmond.
The Herald understands Spanish and British ministers have met informally to discuss Scottish independence.
Crucially, however, Mr Paxman signalled that Madrid jitters should not be allowed to interfere with Scotland's referendum. He wrote: "Spanish concerns should not, of course, influence the course of the constitutional debate in the UK."
A London newspaper on Sunday cited an unnamed UK Government minister saying Spain would seek to block the European Union membership of an independent Scotland.
That claim was denied by the Spanish Government but not before it sparked a flurry of newspaper reports in Madrid, Barcelona and elsewhere. The media reaction forced Spanish Foreign Minister Jose Manuel Garcia-Margallo to say his country would remain neutral in Scotland's constitutional wrangles.
The UK Government is now apparently sending different signals to audiences at home and abroad. British diplomats overseas are playing down the impact of the referendum on Europe. But UK ministers, addressing a domestic audience, are continuing to play up the antipathy of big European players to Scottish independence.
One Coaltion minister, speaking anonymously to The Herald, insisted Spain was worried, despite its formal denials it would veto Scottish EU membership. He said: "It [Spain] would be extremely unhappy about the thought of member states breaking up and would not make that a straightforward process."
Another senior Coalition source last night stressed the Scottish independence referendum was having "serious political implications" for other European governments.
The source was referring to Spain and Italy – where the regionalist Northern League has shown huge interest in Scotland – and to Romania's Hungarian minority.
Nationalist sources stressed their well-developed international links. Many independence movements – from Corsica to the Caucasus – openly refer to Scotland as their pathfinder.
However, newly independent nations outside the European Union have not always received a warm welcome from the rest of the continent.
Spain, Russia, Greece and Cyprus, for example, have all yet to recognise Kosovo, the breakaway Serbian province. Russia has its own national questions to settle – on either side of its border. Greece and Cyprus fear any precedent for international recognition of the Turkish-speaking Northern Cyprus.