WASHINGTON (Reuters) - U.S. regulators have declared insurer MetLife Inc (>> Metlife Inc) so big that its failure could destabilize financial markets, a designation that brings extra regulation.

MetLife said in a statement on Thursday announcing the designation that it was disappointed by the decision, made by the U.S. Financial Stability Oversight Council, and was considering whether to take the regulators to court over it.

The determination "will harm competition, lead to higher prices and less choice for consumers, and ultimately could result in less financial protection for middle-class families," MetLife said.

Congress created the risk council in the 2010 Dodd-Frank law and assigned it to watch for threats to financial stability. The heads of all major federal financial regulatory agencies sit on the council, and U.S. Treasury Secretary Jack Lew leads it.

The group previously declared two insurers, American International Group Inc (>> American International Group Inc) and Prudential Financial Inc (>> Prudential Financial Inc), and one commercial lender, GE Capital (>> General Electric Company), as "systemically" important.

The risk council in September proposed designating MetLife. The company contested the determination, but it said on Thursday it had been notified that the council finalized its decision.

A spokesman for the Treasury Department said the council generally notifies the public one business day after it votes on a final designation.

MetLife has 30 days to seek a judicial review of the decision. None of the other companies designated as risky exercised this option, but MetLife has been more outspoken in its criticism of the council than the other companies.

It has hired top Washington lawyer Eugene Scalia of law firm Gibson Dunn, which many observers took as a sign the company might be preparing a legal challenge.

(Reporting by Emily Stephenson and Sarah N. Lynch; Editing by Steve Orlofsky)

By Emily Stephenson