Six Lloyd's Names issued a writ last week against Euclidian Insurance Services over the purchase of personal stop loss policies for the 1999 and 2000 year of account.
The Stop Loss Recovery Group spokesman David Phillips said: "The six individuals are test cases representing approximately 500 claiming members of the Stop Loss Recovery Group."
He added that the total value of claim "had yet to be determined but would likely be around £20m".
The writ was issued on 3 June and names the defendants as Euclidian Insurance Services, Euclidian Risk Management Ltd and Euclidian Group.
Euclidian Insurance Services managed a company Integer which sold stop loss policies. But Euclidian did not underwrite the policies.
A spokeswoman for Euclidian said the insurer could not comment as "the writ had yet be served" on the insurer.
