The Devolution Unit, created by the Foreign and Commonwealth Office (FCO) in 2012 to deliver abroad the "utmost co-operation", now appears to be at the heart of Westminster's anti-independence drive, amassing hostile reactions from overseas.
It is understood the FCO has contacted the governments of China, Russia, the US, New Zealand, Australia, Canada and the 28 EU nations about the Scottish referendum in a global search for allies who might oppose independence.
One recent cable showed UK embassies being ordered to forward a Westminster paper critical of independence "to their host governments and other local contacts" and then feed their comments back to the Devolution Unit "ASAP".
It would help the Unionist cause if countries raised their concerns about an independent Scotland joining international bodies such as the EU and Nato.
The action is in spite of Prime Minister David Cameron insisting that September's poll is purely "a debate between Scots" - the argument he uses for refusing to debate with Alex Salmond.
The First Minister yesterday issued a fresh challenge to debate to Cameron, saying he had "a responsibility to let people hear his case for the No campaign and for Scotland remaining under Westminster control".
The Sunday Herald has already revealed two examples of Westminster discussing independence with foreign governments.
In December, Downing Street's Scotland adviser Andrew Dunlop and a Cabinet Office official flew to Madrid to discuss the referendum with Mariano Rajoy's government.
With the visit coming soon after Rajoy had undermined the SNP by warning an independent Scotland would be left outside the EU, Alex Salmond accused the Spanish prime minister of plotting a "stitch-up" with Cameron.
The Sunday Herald also revealed how Russia's top news agency had reported Cameron's office was "extremely interested" in getting president Vladimir Putin's support for a No vote.
The SNP last night said the Devolution Unit's behind-the-scenes activity was "a disgrace".
The Unit's head appeared at Holyrood's European and External Relations Committee last July.
Annie McGee, a former vice-consul in Madrid, told MSPs: "Our focus is on working with the Scottish Government, the Welsh Government and the Northern Ireland Executive on their foreign policy interests. I make sure visits overseas run smoothly and that there is the utmost co-operation with our posts. We work with colleagues in the Scottish Government ... to ensure areas of interest are explored as they should be."
Europe Minister David Lidington told MSPs at the same session the Unit was about co-operation. "We are building a working culture between the United Kingdom Government and the devolved administrations in which we co-operate effectively on European policy," he said.
"The Unit ... gives a bit more focused support to that co-ordination role, particularly with regard to the interests of the devolved administrations."
However, official UK government material suggests that, far from advancing the Scottish Government's case, or remaining neutral, the Devolution Unit is actively engaged in promoting Westminster's desire for a No vote.
Last week, after Foreign Secretary William Hague launched the latest in Westminster's Scotland Analysis papers on the problems which could face an independent Scotland, the FCO sent a diplomatic telegram, or "Diptel", message about the document to its staff overseas.
The Sunday Herald has seen its content. It said: "EU Posts are requested to circulate the paper ASAP to their host govts & other local contacts.
"Other posts particularly Washington, Ottawa, Canberra, Wellington & UK Rep Brussels may wish to do so.
"You should refer to previous FCO guidance sent to Posts on how to present the referendum work. "Report back to DEVO UNIT, FCO. Other local reaction (public or private) ASAP."
Other Diptel messages released to the pro-Yes National Collective group under Freedom of Information also show the Unit acting as a clearing house for reactions from overseas governments to Scottish independence.
Angus Robertson, the SNP's Westminster leader, said: "In public David Cameron has pledged that the referendum is for people in Scotland.
"In private he's using UK diplomats around the world to support the 'no' campaign.
"Governments internationally have said they won't get involved in this democratic debate in Scotland.
"It's a disgrace that the Prime Minister is breaking his word, encouraging foreign interventions while running scared of a debate with First Minister Salmond."
A Westminster source said the SNP's attack was "frankly quite ludicrous", as Salmond was in regular touch with other governments, and it was routine for the Westminster government to share information abroad, "especially about issues that have ramifactions outwith the UK".
A Downing Street spokesman added: "The SNP can debate about debates all they like. We are getting on with informing the debate with detailed analyses so that people can decide."