The Treasury's most senior mandarin, Sir Nicholas Macpherson, is facing questions about his relationship with Labour MP and Better Together chairman Alistair Darling after he last week put his own concerns about a currency union into the public domain.

Civil servants have to be politically impartial, but Macpherson, the permanent secretary to the Treasury, wrote that a currency union was "fraught with difficulty" and "strongly" advised against one.

In a freedom of information request from November 2012, the Treasury stated that Sir Nicholas meets Darling "socially from time to time".

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A Yes insider said: "This raises the question of whether Mr Darling was involved in the orchestration of last week's events, including the unseemly spectacle of Ed Balls parroting the words of a right-wing Tory chancellor trying to lay down the law to Scotland."

A Treasury spokesman downplayed the meetings, saying that Darling and Macpherson worked together when the MP was chancellor.