Revealed: how UK diplomats helped Darling's pro-union US charm offensive

The former Chancellor's trip had been arranged and paid for by the International Monetary Fund, which had invited him to speak at a seminar. However, on the margins of the IMF event, the UK Government also arranged a series of meetings about the Scottish independence referendum between Darling, the media and think-tanks.

New material released by the FCO under Freedom of Information legislation shows that the British Embassy in Washington "facilitated introductions" with journalists at nine media outlets: Channel 4, the BBC, ITN, the Financial Times, the Telegraph, the Times, the Independent, the Guardian and the Economist.

Darling was accompanied by a British diplomat when he met the reporters for a briefing.

The embassy also facilitated contacts with four powerful Washington-based bodies specialising in defence and international relations, despite Darling's background being in economics and finance.

Two were divisions of the school of foreign service at Georgetown University: the BMW Center for German and European Studies and the Mortara Center for International Studies.

In 2012, the BMW Center hosted a speech by the then Liberal Democrat Scottish Secretary Michael Moore on "Scotland in the World", in which he set out his case for maintaining the Union. Moore's speech was published in full, but there is no public record of any contact with Darling.

The FCO also played matchmaker between Darling and think-tank the Centre for Strategic and International Studies (CSIS), and the Center for Transatlantic Relations at John Hopkins University.

CSIS has published a series of articles on the impact of Scottish independence on the Trident nuclear weapons system based at Faslane, which the SNP wants to expel by 2020 after a Yes vote.

Darling's speech at the Center for Transatlantic Relations was made in private.

At the time, Better Together confirmed Darling had taken part in "a range of referendum-related engagements" in the US, but claimed he received embassy support because he was an Edinburgh MP.

The UK Government also said in April that it was "entirely appropriate" for the FCO to help.

However, while it is standard protocol for UK and Scottish ministers to receive local embassy support on foreign visits, Darling is not a minister, but rather head of a political campaign.

Westminster justified its support on the basis that Darling was advocating the UK Government's policy of keeping the union.

The SNP last night demanded full disclosure of the FCO's support for the No campaign.

SNP Westminster leader Angus Robertson said: "This new information shows that the FCO has gone to huge lengths to facilitate Alistair Darling to brief against independence.

"That UK diplomatic resources and staff are used to benefit the head of the No campaign, a backbench MP, is utterly unacceptable.

"The UK Government must urgently publish full details of the purpose of Mr Darling's engagements with the media and thinktanks. It has a duty to ensure that it is not abusing taxpayers' money for political means.

"There must now be a full and frank disclosure of how much has been spent assisting Mr Darling and transparency about what was discussed at private meetings held during the trip."

The Sunday Herald also revealed this year that the FCO's Devolution Unit, which was ostensibly set up to help the Scottish Government with its work abroad, was being used to distribute pro-Union messages to foreign governments around the world, and then gather feedback.

A UK Government spokesman said: "The FCO regularly provides support to UK parliamentarians from all parties during overseas visits.

"It was entirely appropriate for the FCO to provide assistance to the former chancellor during his visit to the United States.

"UK embassies provide considerable support for Scottish Government ministers when they are overseas, and supported SNP MP Angus Robertson on his visit to Oslo last month."