Lloyds Action Now (LAN) is the action group acting on behalf of small shareholders following Lloyds' failed merger with HBOS during the recession.
It has produced details of a claim and filed them with both the Bank of England and the treasury, the Telegraph reports.
The basis of the allegation is that action taken to assist HBOS included illegal state aid.
Governor of the Bank Mervyn King had earlier revealed that the body gave emergency loans worth Â£61.6 billion to HBOS and the Royal Bank of Scotland.
However, LAN has claimed that the provision of emergency liquidity assistance, which is a rescue fund provided by the Bank at the Treasury's request, that was given to HBOS in October 2008, included a selective grant of state aid - as defined by the European Union constitution.
The action group also described the aid as "extraordinary" and suggested that the package constituted rescue aid because HBOS was insolvent.
A writ from the organisation stated: "The aid remained illegal for the entire time of its grant, i.e. until it was repaid on January 16, 2009."
There are estimated to be around 800,000 small investors who owned equity in Lloyds TSB prior to the merger - and LAN is currently attempting to distribute 300,000 letters to shareholders to keep them updated on the situation.
The Treasury and the Bank have 90 days to respond to the writ and are expected to challenge the claims made.
Bloomberg recently reported that the UK government is looking to offload its stakes in Lloyds Banking Group in an attempt to create a budget windfall ahead of the next election.
The news agency predicted that such a sale would likely go ahead next year.
By Gary Cooper