PORTSMOUTH, N.H. – A major failure of an underwater electrical cable is expected to keep a drawbridge over Portsmouth Harbor closed to road traffic for up to five months, the Transportation Department said Thursday.

The Sarah Mildred Long Bridge, which carries U.S. Route 1 bypass traffic over the Piscataqua River between Portsmouth and Kittery, Maine, had a power outage on Saturday.

The department determined that the damage is below the waterline in a submarine cable that carries electricity to power key bridge lift motors. The specialized cable needs to be replaced to get the bridge working again.

It’s expected to take up to 120 days to make the cable; repairs could take up to 30 more days.

The department estimates it will cost $1 million for the repairs.

Crews raised the bridge Tuesday using an emergency generator and planned to leave it in the up position so boats aren’t blocked from traveling up and down the Piscataqua River.

“We recognize that leaving the Sarah Long Bridge in the raised position for a long period of time will present a significant inconvenience to motorists and some businesses in the Portsmouth area,” said Carol Murray, transportation commissioner. “Unfortunately, this is not an easy problem to fix. Working with the Maine Department of Transportation, we will do everything we can to expedite the repairs and minimize the impact as much as possible.”

Detours will continue to guide motorists to the Interstate 95 and Memorial bridges. Construction work also is under way on a turnaround near Albacore Park on the Route 1 bypass that will allow vehicles to more easily access businesses along the highway, the department said. The turnaround is expected to be open to traffic on Saturday.

The Sarah Mildred Long Bridge was built in 1940 and has carried an average of about 11,000 vehicles a day across the Piscataqua River.