Bristol-based practice Ringrose Wharton is to pursue personal pensions mis-selling claims on behalf of 150 members of Unison, the UK's largest trade union.
The additional work means Ringrose is investigating about 450 potential actions, including claims from members from two other trade unions, the Royal College of Nursing and GMB.
Shacklocks in Mansfield, Girlings in Kent, and Reynolds Porter Chamberlain in London are also acting for victims of the mis-selling scandal.
A large number of City law firms are involved on the other side of the fence, representing life insurance companies and independent financial advisers which may have wrongly advised people to pull out of company pension schemes in favour of personal pensions at the end of the last decade.
The City firms include Herbert Smith, Simmons & Simmons, Lovell White Durrant, Cameron Markby Hewitt, Pinsent Curtis and Clifford Chance.
Ringrose partner Philip Ryley said a number of claims for mis-selling have already been settled out of court and he expected more to follow.
Ryley said: "The first cases to come to court will be important because they will establish the extent of duty of care and how compensation is calculated."
Claims that cannot be resolved with life insurance companies or independent financial advisers should reach the High Court at the end of the year.
Ringrose senior partner Robert Wharton said: "The mis-selling of personal pensions will undoubtedly mean many other law firms will be approached by their clients to provide advice and if necessary to litigate on their behalf."
City regulators have ordered the life insurance industry to conduct its own review of the mis-selling scandal but the timetable has slipped, prompting unions to go down the route of legal redress. Another reason for resorting to the courts is scepticism over the review process, which some claimants argue will involve life insurers investigating themselves.
Wharton said: "The life industry has put in place a review programme which relies on the mis-selling company to investigate its own practices. This review is way behind schedule and leaves tremendous room for conflict of interest."
Unison's national pensions officer Glyn Jenkins said: "We have given this step [legal action] considerable thought. We are dissatisfied with the progress being made by the life and pensions industry.
"Self-regulation of the life insurance industry is clearly not working and this is borne out by the growing number of actions over the last few years."