Britain was seriously disrupted yesterday by its first national railroad strike in a quarter century, as 20,000 engineers walked out on government-owned British Rail.

The primary victims of the strike, Washington Post correspondent Leonard Downie Jr. reported, are big-city commuters, while automobile traffic and freight-hauling were already hampered by snow, ice and fog.

The engineers, who will not return to work until Friday, intend to repeat the two-day stoppage next week. They also are refusing to work overtime or Sundays. British Rail has threatened to close down indefinitely.

British Rail refused to give the engineers a scheduled 11 percent rise after their union failed to agree to seven- to nine-hour shifts that would enable each engineer to operate more trains than on conventional eight-hour days. It is typical of productivity disputes here during the country's worst recession since the 1930s.

A gradual recovery from the recession widely forecast for this year could be endangered by a long rail strike as well as by looming disputes in the government-owned coal mining and steel industries. These industries have a "triple alliance" for mutual aid in prolonged disputes.

There have been significantly fewer strikes and fewer workdays lost through labor disputes here during Prime Minister Margaret Thatcher's tenure than during the previous decade.