The Canadian government announced today that it has selected a jet fighter manufactured by McDonnell-Douglas Corp. of St. Louis for a $1.9 billion effort to modernize its Air Force in the 1980s.

The announcement said Canada would purchase a minimum of 137 F18A twin-engine aircraft with delivery expected to begin in 1982.

The selection of the F18A over the F16, which is manufactured by General Dynamics Corp., also of St. Louis, ended a five-year-long debate involving regional and business interests as well as aircraft manufacturers.

In reaching the decision, the first major one taken by the Liberal government elected in February, Prime Minister Pierre Trudeau's Cabinet ignored a last-minute plea by Liberals from Quebec, saying a decision in favor of McDonnell-Douglas might aid the separatist movement that wants to split that French-speaking province from the rest of Canada.

The government hopes to use industrial contracts offered to Canada by the aircraft manufacturers to enhance this country's industrial capacity, and the comparative benefits offered by competing companies have long been a subject of debate.

Recently, Quebec politicians have argued that the General Dynamics aircrafts would offer greater industrial benefits to their province. Selection of the McDonnell-Douglas plane, they said, would give further ammunition to Quebec Premier Rene Levesque to use in his drive for Quebec independence.

A federal government spokesman said today that the government believes McDonnell-Douglas' industrial benefits program "represents the best deal for Canada." He denied that General Dynamics' contract offer would have been more valuable for Quebec.

The competition, which once involved five aircraft companies, has been dragged out since 1975 because of delays caused by two national elections in the last year and disputes over which company offered the best industrial benefits package.

Delivery of the aircraft to Canada is expected to begin in the latter half of 1982, and will continue at the rate of two aircraft a month until 1989.