A large hole in the northbound lane of Montgomery County's Cabin John Bridge caused a 12-mile backup on the Capital Beltway during the peak of morning rush hour yesterday, forcing police to close the bridge to motorists until the hole could be patched with a steel plate, Maryland state police said.

Virginia state police said the hole caused a three-mile backup on I-66 during peak morning traffic.

Police and transportation officials said they anticipate no further traffic problems from the hole.

The "deck puncture" measured 3 feet by 5 feet, according to Edward Wrzesinski, resident maintenance engineer for Maryland's State Highway Administration.

Wrzesinski said it tood a crew nearly two hours to lay the plate over the hole. The hole will be repaired during an evening next week when traffic is light, he said.

The Cabin John bridge, which carries about 20,000 vehicles a day, is scheduled for major repairs next year or in early 1985, Wrzesinski said. The entire 20-year-old deck will be replaced then, he said.

"A deck puncture is not an uncommon thing," according to Edward Payne, office engineer for maintenance with the State Department of Transportation.

Payne explained that such holes occur when a weak spot develops underneath the roadway and the bottom gives way. "It comes from underneath and then a pothole occurs on the surface after a piece has fallen out," Payne said.

"A person couldn't fall through it. You could only blow out tires or bend a rim," Wrzesinski said.