Fund managers who invested customers' money in Royal Bank of Scotland's disastrous £12 billion rights issue in June 2008, but who failed to alert clients to the existence of a shareholder-led legal action against the bank, are vulnerable to being sued by their clients, according to a leading Scottish lawyer.

In an opinion prepared for an independent financial adviser representing high net-worth clients before proceedings next month at the High Court, John Campbell QC said any fund manager who steered clients into the controversial rights issue, but failed to alert investors to the RBS Shareholder Action Group's existence, and/or failed to take part in the group's action, would be vulnerable to being sued – but only if the action proves successful.

Campbell added: "A failure to draw clients' attention to the existence of RBSSAG and its intentions is prima facie negligent conduct and therefore actionable."

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He said that any fund manager, which he takes to include asset-management firms, nominee accounts, brokers, wealth managers and IFAs, who failed to inform and advise their client of the existence of the action by the cut-off date of April 2014 "is likely in my opinion to be seen to have been negligent and have a case to answer".

Campbell acknowledged that different fund managers and intermediaries have a range of types of contract with their clients, but said contractual terms and conditions "will be subsumed under the overarching principle of the duty of reasonable care".

The action group alleges that, through "misleading statements" and "critical omissions", RBS and four of its ex-directors – Fred Goodwin, Sir Tom McKillop, Johnny Cameron and Guy Whittaker – misled investors into putting £12bn into the rights issue. Despite reassurances at the time, the Edinburgh-based bank was near bankrupt four months later, its shares near-worthless.

The RBSSAG, advised by law firm Bird and Bird and Philip Marshall QC, is putting the finishing touches to proceedings which will be issued in London's High Court in mid-December. The action group recently put £15 million of adverse costs cover into the court.

So far 12,000 individual investors and 100 institutional investors have joined the legal action.

RBS declined to comment.