Jean-Jacques Servan-Schreiber, the founder of newsweekly journalism in France and once an iconoclastic critic of French society, is resigning as head of L'Express after selling 45 per cent of the magazine to London-based financier James Goldsmith today.

Servan-Schreiber, who wrote "The American Challenge" a decade ago to exhort European business and industry to adopt more competitive attitudes and methods to keep up with American business, has told associates he will devote himself to politics and to supporting the government of President Valery Giscard d'Estaing.

A brief communique on the sale of about half of the shares owned by Servan-Schreiber and his family did not list a price. Informed sources said Goldsmith paid $5 million to $6 million.

The purchase is another indication of the interest of Goldsmith, who holds dual French and British nationalities, in building a netwoek of press holdings. After failing in a bid to buy the London Observe last year, he bought into Britain's Beaverbrook press and Evening Standard in Lon-group, which includes The Daily Exdon.

Industry sources say that one of his conditions for injecting fresh capital into L'Express is that the magazine will help him launch a daily financial newspaper in Paris early next year.

Goldsmith is also attempting to shift into France more of his business activity, which is centered on a food-processing and distribution operation that is one of the four largest in Europe.

At a two-hour closed meeting last night, Servan-Schreiber told staff representatives that there would be no immediate changes in the way the magazine would be run beyond his resignation as president of L'Express group, a post between chairman of the board and publisher.

A member of the National Assembly, head of the regional legislature in Lorraine and now a special representative of Giscard in administrative reform, Servan-Schreiber is known to feel that only the reformist programs of his small Radical party can prevent a destructive clash between France's increasingly polarized rightist and leftist forces.

The deal also brings the organization badly needed cash. Founded in 1953, L'Express has seen its circulation drop from 620,000 to 500,000 in two years, as a new and more conservative weekly, Le Point, has mounted a strong challenge to L'Express, which has consistently given highly favorable coverage to Giscard and his closest political associates.