A company headed by former General Motors, executive John Delorean will build a $125 million sports car plant in Belfast, the British government confirmed last week.

With Delorean beside him, Northern Ireland Secretary Roy Mason told a news conference that the car should be coming off the assembly line here in 1980.

The Puerto Rican government threatened legal action against Delorean after learning he had signed an agreement to shift a planned $96 million assembly plant from a former Air Force base at Aguadilla, Puerto Rico, to Northern Ireland.

Delorean's switch came only hours after the U.S. Conference and Agriculture departments approved $40 million in loans for the Puerto Rican government was to provide $20.65 million in loans and grants and Delorean was to supply $25 million.

here have been persistant reports that Delorean was having difficulty completing his part of the financing for the Puerto Rican facility. The announcement that $100 million in financing had been obtained for the Dumurray plant indicated that substantially more assistance was available with the shift to Northern Ireland.

Delorean said the Northern Ireland Development Agency and Commerce Department was able to conclude arrangements for the deal quickly, including clear title to the assembly site. Construction on the 550,000-square-foot plant will begin in 60 days, Delorean said.

About 20,000 of the new sports cars are expected to be assembled in the first year, rising to an annual output of 30,000 cars in the third year. The Northern Ireland facility will employ about 2,000, the company said.

Delorean will maintain a U.S. headquarters, and the company said about 20 percent of the new cars will be marketed in Canada and Europe and the remainder in theUnited States.