BHP Billiton has hired three former advisers to Canadian prime ministers, including one who worked for Stephen Harper, the current prime minister, as it seeks to build political support for its hostile takeover of Potash.

BHP, the world's biggest miner, hired lobbyists Michael Coates, William Pristanski and Bruce Hartley last month, according to Canada's lobbyist registry. Lobbyists who contact Canadian government officials are required by law to identify themselves and who they work for, with the details made public online.

The Anglo-Australian miner's chief executive, Marius Kloppers, is in Canada this week seeking support for its Potash bid, which would require government approval. BHP has offered $39 billion for Saskatchewan-based Potash, which is the world's biggest fertilizer producer.

Coates, who serves as president and CEO of Hill & Knowlton Canada, led a team that prepared Harper for debates during the past three elections, according to Hill & Knowlton's website.

Pristanski has held numerous public offices, according to the registry, and assisted former Prime Minister Brian Mulroney during the 1980s. Hartley was a long-time personal assistant to former Liberal Prime Minister Jean Chretien.

The lobbyists say in their registration documents that they intend to, or have, contacted numerous federal departments as well as the Prime Minister's Office.

Asked in a Business News Network interview on Tuesday if BHP would keep the lobbyists busy, Kloppers said, "absolutely".

Potash last month hired lobbyists Brian Klunder and John Capobianco of Fleishman-Hillard and Lawson Hunter of Stikeman, Elliott, according to the registry.