He soon became involved in trade unionism, and in 1929 became the full-time assistant general secretary of the International Mercantile Marine Officers' Association.
[1][2] Threatened by World War II, Becu escaped to the UK in 1940, continuing in his trade union posts.
[1][2] In 1944, Becu was able to return to Belgium, where he and some surviving comrades began re-establishing the trade union movement.
In 1950, he was appointed as the ITF's general secretary, giving up his posts in Belgium, and in 1953 he was additionally elected as president of the International Confederation of Free Trade Unions (ICFTU).
His term of office ended in 1957, but he remained involved with the federation, and was elected as its general secretary in 1960, serving until his retirement in 1967.