Prime Minister of Thailand Rajanubhab took over this role. In fact, the office most considered the precursor of that of the prime minister was the ancient office of "Samuha Nayok" (สมุหนายก), which was run by an "Akkhra Maha Senabodi" (อัครมหาเสนาบดี) or "chief minister in charge of civilian affairs". The first Prime Minister of Siam was Phraya Manopakorn Nititada, a judge. The title of the office was changed from "Prime Minister "of Siam"" to "Prime Minister "of Thailand"" in 1945 and then permanently with the renaming of Siam to Thailand in 1949. For most of its existence the office has been occupied by Army leaders;
Acting prime minister is on vacation, or is in ill health. In the early 20th century, when travel by ship was still the norm, it was not uncommon for there to be an acting Prime Minister for months on end. The deputy Prime Minister is usually designated as the acting Prime Minister, although another senior member of the government may fill the role if both the Prime Minister and deputy Prime Minister are unavailable. An acting Prime Minister is required quite frequently – for instance, between 3 December 2007 and 23 February 2009 (during Kevin Rudd's first term), Julia Gillard acted as Prime