Britain will buy an eighth C-17 military transport aircraft from U.S. firm Boeing, Prime Minister David Cameron told parliament on Wednesday, citing the need to have enough air capacity to evacuate civilians from war zones.

Cameron said the Ministry of Defence, which is cutting 8 percent in real terms over four years from its 34 billion pound budget, had found extra cash to buy the plane, which an aide close to him said would cost 200 million pounds.

I can announce today that because the Ministry of Defence's finances are now better run and better managed and because we have found savings, we will be able to purchase an additional C-17 for the RAF (Britain's military air force), Cameron said.

Britain's existing fleet of C-17s is used for airlifts and transporting heavy bulk.

(Reporting by Mohammed Abbas; Editing by Matt Falloon)