Nov 29 (Reuters) - Canada is set to announce on Thursday a multi-billion dollar sole-source contract for Boeing Co’s P-8 Poseidon maritime patrol aircraft to replace the country’s military surveillance planes, a senior government source familiar with the matter told Reuters.

An announcement would end months of speculation and follow a challenge from Canadian private planemaker Bombardier which called for an open competition so it could submit a bid using its large-cabin Global business jets.

The source spoke anonymously about the private matter.

In March, the Canadian government said it was looking at buying 16 P-8A Poseidons to replace its aging fleet of CP-140 Aurora aircraft, arguing that Boeing's aircraft is the only "currently available" one to meet all its requirements.

A draft agreement between the governments of Canada and the United States for the sale of the Poseidon aircraft and associated training and equipment is set to expire on Thursday.

Bombardier said it would only comment following official notification from the government. Boeing declined comment on Wednesday. (Reporting By Steve Scherer in Ottawa; Additional reporting by Valerie Insinna in Washington; Writing by Allison Lampert in Montreal; Editing by Chris Reese and Sonali Paul)