Since 1963.
Singapore’s stamps were valid for postage (and vice-versa) in Malaya / Malaysia via the Malayan Postal Union agreement on 01 January 1934. The Pan Malaya Postal Agreement continued this arrangement from 09 August 1949 until 31 January 1967. From 15 May 1964, North Borneo (Sabah) and Sarawak stamps were also allowed to be used in Singapore and vice versa.
