ALEX Salmond last night demanded a criminal investigation into alleged "insider dealing" at the Treasury on the eve of the referendum, when it revealed the Royal Bank of Scotland would relocate to London in the event of a Yes vote.

Following the refusal of the head of the civil service to investigate the leak, the First Minister has written to the head of the Financial Conduct Authority, the Chief Constable of Police Scotland and the Commissioner of Police for the City of London urging action over the alleged leaking of market-sensitive information.

He said he was concerned "a criminal offence may have been committed" when the Treasury briefed the media on September 10 about RBS contingency plans to move, while the RBS board was still discussing the matter, and before the bank had made a statement to the financial markets.

A Treasury email sent to journalists at 10.16pm, around 25 minutes before the RBS board finished its meeting, said: "As you would expect, RBS have also been in touch with us and have similar plans [to Lloyds] to base themselves in London."

The news that a major Scottish employer such as RBS, which is 80% owned by the taxpayer, planned to relocate its "brass-plate" headquarters because the UK offered it greater financial protection than an independent Scotland was a blow to the Yes campaign.

The next day, Salmond attacked the Treasury for its behaviour, and accused the BBC of "bias" for the way it reported the RBS move. Salmond also demanded an inquiry by Cabinet Secretary Sir Jeremy Heywood, but Heywood swiftly rejected the idea.

In his new letter, Salmond said he was concerned there had been "improper disclosure of market sensitive information to another person which is tantamount to insider dealing".

He said: "I view the actions of HM Treasury with the utmost seriousness. It is essential that an independent investigation takes place in order to protect the integrity of the UK financial system."

Opposition MSPs said Salmond was embarrassing himself and warned against wasting police time.

The new complaint echoes one made by Salmond's friend, the Edinburgh financier and Independent Midlothian councillor Peter de Vink. Last month, de Vink asked Police Scotland, the FCA, The City of London Police and the Lord Advocate to investigate the Treasury's actions.

He told the Sunday Herald the FCA and the Lord Advocate had referred him to the police. The police, he said, had done nothing.

At a meeting last week with Salmond, de Vink raised the matter with the First Minister, prompting the new letter.

Tory MSP Alex Johnstone said: "It's embarrassing for Scotland to watch the First Minister flog a dead horse in this way.

"It's everyone's right to file a complaint if they suspect criminal wrongdoing. But they should think carefully, because they don't want to stand accused of wasting police time."

LibDem leader Willie Rennie said: "Instead of picking over lost battles past, he should retire gracefully."