L&G is understood to be a key mover behind a case being prepared by American litigator Quinn Emanuel Urquhart & Sullivan over a £12 billion fundraising by RBS in April 2008, months before the bank was rescued with a £45bn taxpayer bail-out. At least one other major UK investor is understood to be backing the Quinn Emanuel case.
Two other claims for compensation have already been submitted to the High Court in London.
The Royal Bank of Scotland Shareholder Action Group, which represents around 12,000 small investors, has hired London law firm Bird & Bird to seek up to £4bn in compensation. Another 21 UK and foreign institutions, including Strathclyde Pension Fund, are backing another claim led by Stewarts Law for hundreds of millions of pounds.
Both cases are understood to focus on alleged failings in RBS's disclosures to shareholders, which the claimants said failed to meet the requirements of the Financial Services and Markets Act 2000.
A spokesman for L&G declined to comment on its involvement in the latest case. Quinn Emanuel partner Martin Davies, an expert in shareholder disputes who is understood to be playing a leading role in the case, also declined to comment.
It is understood that Quinn Emanuel lawyers participated in a case management conference in London with counterparts from Bird & Bird and Stewarts Law in July. A second meeting due next week will determine whether, and to what extent, the claimants will have to combine their cases.
A spokesman for RBoS Shareholder Action Group declined to comment, however a source close to the organisation said L&G's involvement "adds credibility" to the legal claims.
The size of L&G's potential claim is not known. However, it remains the third largest shareholder in RBS.
Royal Bank of Scotland declined to comment.