Scotland's top law officer has rebuked UK Business Secretary Vince Cable after the politician called on prosecutors to reach a decision "as quickly as possible" about possible action against the directors of Royal Bank of Scotland (RBS) at the time of the financial giant's collapse.

Lord Advocate Frank Mulholland voiced disappointment that Mr Cable had written to the Advocate General on the issue - a man said to have no role in the investigation.

Mr Mulholland said it would be "unfortunate" if the move by the Business Secretary was to be construed as an attempt to interfere with prosecutorial decision-making.

In his letter to Advocate General for Scotland, Lord Wallace, the Business Secretary said there was "considerable public concern" about the actions of the lender's leadership, which left RBS needing a £45.5 billion taxpayer-funded rescue.

The matter was referred to Scottish prosecuting authority the Crown Office and Procurator Fiscal Service (COPFS) in January last year following a damning report into the bank's failings by the Financial Services Authority (FSA).

Mr Cable asked for an update on the progress of the case but insisted he was "not seeking to influence the outcome" of the legal process.

The Business Secretary told his fellow Liberal Democrat in the letter: "Following the release of the FSA's report into the failure of RBS I sought legal advice on what if any enforcement action was appropriate and was advised that the Crown Office and Procurator Fiscal Service should consider a possible prosecution.

"Given that this matter was referred to them in January 2012, I am very keen for a decision to be reached as quickly as possible in order to maintain public confidence in the efficiency of the decision-making process.

"I am fully aware that the decision whether or not to prosecute rests with the Crown Office and Procurator Fiscal Service as the relevant independent prosecuting authority.

"I want to be clear that I am not seeking to influence the outcome of this process. However, public and media interest in the banking sector and RBS have not dissipated.

"There have been numerous questions about what steps can be taken to address concerns. In particular this has focused on the prosecution and possible disqualification of former directors in appropriate cases."

The Business Secretary said he appreciated the complexities of the case but asked for an update on progress and the likely timeframes for a decision.

The Lord Advocate responded by saying the investigation was complex and involved a large amount of material.

Mr Mulholland said: "I am disappointed to hear that the Secretary of State for Business Innovation and Skills (BIS) has written to the Advocate General in such terms. The Advocate General has no role in the investigation or prosecution of crime in Scotland.

"It would be unfortunate if this were to be construed as attempted interference with independent investigation and prosecutorial decision-making by the law officers.

"In the wake of widespread public concern about reported issues in the banking sector, I instructed an investigation into allegations involving the Scottish banking sector.

"That investigation has been led by the COPFS Serious and Organised Crime Division. The investigation is complex and ongoing and the volume of material being considered is vast."

In a statement, Mr Mulholland said Crown Office officials have kept BIS officials advised of progress throughout the investigation.

"Public confidence in Scotland into the investigation of allegations of criminality occurring here is best maintained by thorough independent investigation by the COPFS and not communicated through the media," he said.

"If the Secretary of State contacts me directly I would be happy to brief him on progress to date and timescales."

RBS was brought to its knees by "multiple poor decisions" and a £50 billion "gamble" on buying Dutch bank ABN Amro, the FSA report found.

The report shone a light on the poor relations between the FSA and RBS, and said chief executive Fred Goodwin's "assertive and robust" management style was flagged as a potential risk as early as 2003, four years before the disastrous ABN Amro deal.

In January 2012, a month after the FSA report, Mr Goodwin was stripped of the knighthood he was awarded for services to banking.

A BIS spokesman said: "Vince Cable wrote to the Advocate General out of courtesy as a fellow minister. He has also copied this to the Lord Advocate as well and looks forward to his reply."