Maryland's highest court ruled yesterday that homeowners may sue the Washington Suburban Sanitary Commission for damages from broken water mains, sewer pipes and other types of problems that the water and sewer agency causes by negligence.

The WSSC has successfully defended itself against such suits for the past three years by claiming it is a state agency, and may not be sued for negligence.

The ruling opens the way to a host of suits by homeowners and others for damages caused by sewer backups, water breaks, auto accidents involving WSSC vehicles, open manholes, contract disputes and others, as long as the WSSC can be demonstrated to have caused the problem by negligence.

The ruling is expected to cost the water and sewer agency millions of dollars.

"You're going to have substantial amounts of money involved. It's going to have a severe impact on the commission," said Paul J. Hefferon, the WSSC's general counsel. "It's not nice. It's not nice," he said.

Hefferon said that the agency first began claiming immunity from damage suits in 1976, when the Court of Appeals held that a state-created bicounty agency, the Maryland-National Capital Park and Planning Commission, was immune from damages in such cases. Yesterday, the court stripped the WSSC of that defense, finding that the agency's own code included, in effect, a waiver of immunity.

The suit before the Court of Appeals consolidated three separate claims for damages that had been brought against the WSSC. One suit was a claim for damages for goods and inventory lost when a WSSC water main burst and flooded a basement. In another case, the Chesapeake and Potomac Telephone Co. of Maryland, sued for money to compensate it for the amount spent repairing telephone cables cut by a contractor working for WSSC.

In the third case, the Liberty Mutual Insurance Co. sued to recover money it had paid a policy holder whose house was destroyed by fire. The insurance company claimed that the WSSC's negligent maintenance of fire hydrants caused fire department pumps to become clogged with rocks and debris that came from the hydrants.

Hefferon said several damage suits are pending against the agency, including at least one asking for $12 million. "If you put 20 more together at $100,000 each, you're going to have substantial amounts of money involved," he said.

Past suits, in which the WSSC had successfully defended itself by claiming immunity, might be brought again, he said.