SIR Nicholas Macpherson, the Whitehall mandarin whose intervention into the Scottish independence referendum campaign sparked controversy, has been awarded the Order of the Bath in the Queen's Birthday Honours.

Sir Nicholas, 55, has received the honour for "public service", according to the official citation.

The Treasury's Permanent Secretary hit the headlines in February 2014 when he took the unusual step of publishing his private advice to Chancellor George Osborne, strongly opposing currency union between an independent Scotland and the rest of the UK; one of the SNP Government's primary proposals.

At the time, John Swinney, the Deputy First Minister, insisted Sir Nicholas had "crossed the line" of civil service impartiality but the Treasury was adamant that the advice had been "completely impartial".

In a speech in January, the civil service chief defended his action, arguing in such an "extreme" case as last year's referendum, in which "people are seeking to destroy the fabric of the state" and to "impugn its territorial integrity", the normal rules of civil service impartiality did not apply.

Yet two months later, UK Government Ministers were heavily criticised by the Commons Public Administration Committee for allowing Sir Nicholas's private advice to be made public, insisting it had been done for political reasons and had "compromised the perceived impartiality of one of the UK's most senior civil servants".

Alex Salmond, the former First Minister, accused Sir Nicholas of "unacceptable conduct", noting how "a Government department, like a fish, rots from the head down".

Following the Commons report, he made clear the mandarin's time was "well and truly up", adding: "My advice to him is...do the honourable thing and resign".

The Order of the Bath is mainly awarded to officers of the Armed Forces as well as to a small number of civil servants for service, which merits "peculiar honour or reward".

The title of the Order is late medieval in origin and relates to the ritual washing, inspired by baptism and meant to purify the soul, which formed part of a knight's preparations for the conferment of knighthood.

The earliest official mention was after the coronation of William I, when Geoffrey, the Count of Anjou, aged just 15, took part in a bathing ceremony before being knighted.