The treaty establishing the Benelux Customs Union was signed on September 5 , 1944 , by the governments in exile of the three countries in London , and entered into force in 1948 . In 1944 , the three countries signed the London Customs Convention , the treaty that established the Benelux Customs Union . Ratified in 1947 , the treaty was in force from 1948 until being supplanted by the Benelux Economic Union .