Cameron wrote in the Thursday edition of The Daily Telegraph that the Trident nuclear missile system served as the "ultimate insurance" against a nuclear attack.
He said British forces needed a nuclear deterrence now as much as it did at the height of the Cold War. With North Korean aggression, continued defense was needed now more than ever.
"Last year North Korea unveiled a long-range ballistic missile which it claims can reach the whole of the United States," he wrote. "If this became a reality it would also affect the whole of Europe, including the U.K."
Blog 38 North from Johns Hopkins University reports that commercial satellite imagery shows North Korea started construction work at a plutonium reactor at the Yongbyon nuclear complex. North Korea in 2007 shuttered the facility as part of multilateral nuclear negotiations.
South Korean Defense Minister Kim Kwan-jin said Thursday that North Korea has moved missiles to the east coast in preparation for a launch.
"It cannot reach the U.S. mainland," he was quoted by China's official Xinhua news agency as saying. "The missile seemed to be moved for the test-firing or military drill purpose."
White House spokesman Jay Carney said the U.S. government was monitoring the situation on the Korean Peninsula.
"Bellicose rhetoric" from the North Korean government "represents a familiar pattern (that) we've seen over the past several administrations," he said.