A federal appeals court Tuesday vacated class certification in six key cases in the massive litigation over dot-com era initial public offerings — a potentially devastating setback for plaintiffs in the biggest consolidated securities class action in U.S. history.

The decision by a three-judge panel of the 2nd U.S. Circuit Court of Appeals leaves in doubt whether plaintiffs will ever be able to certify a class against IPO underwriters in the more than 300 cases that make up In Re IPO Securities Litigation. It may also help to unravel a pending $1 billion settlement agreement reached between IPO issuers and the plaintiffs, lawyers involved in the case say.