FARNBOROUGH, England (Reuters) - British Prime Minister David Cameron is likely to finalize multi-billion dollar deals with Boeing <BA.N> on Monday to buy nine P-8A Poseidon maritime patrol planes and to upgrade 50 Apache helicopters, two industry sources said on Sunday.

Cameron is expected to announce the deals at the opening of the Farnborough Airshow in southern England.

Spokespeople for the prime minister and Britain's Ministry of Defence declined to comment.

Britain announced its intention to buy the submarine-hunting P-8A planes in November to plug a gap in its defenses that has existed since 2010, when it ditched the Nimrod, built by Britain's BAE Systems <BAES.L>.

Cameron said last month he would step down as prime minister once his ruling Conservative Party had chosen a successor, following a referendum in which Britain voted to leave the European Union. Cameron had campaigned for staying in the EU.

Britain's Minister for Defence Procurement Philip Dunne said on Friday the "Brexit" vote did not change the country's commitment to the NATO military alliance and that it was "not retreating into its shell".

(Reporting by Tim Hepher and Kylie MacLellan; Writing by Mark Potter; Editing by Adrian Croft)