FARNBOROUGH, England (Reuters) - Britain announced on Monday the completion of multi-billion dollar deals with Boeing <BA.N> to buy nine P-8A Poseidon maritime patrol planes and to upgrade 50 Apache helicopters.

The Ministry of Defence made the announcements at the opening of the Farnborough Airshow in southern England, the aerospace industry's premier showcase.

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>.

Defence Secretary Michael Fallon said they would provide protection to the country's nuclear weapons and aircraft carriers.

He did not give an exact price for the order, but said the development of maritime patrol aircraft, including infrastructure and training, would cost the country 3 billion pounds ($3.9 billion) over the next decade.

The deal involving the 50 AH‑64E Apache helicopters is worth $2.3 billion, the ministry of defense said in a statement.

($1 = 0.7762 pounds)

(Reporting by Conor Humphries; Editing by Mark Potter)