LONDON — In a significant reversal of British government policy, Prime Minister David Cameron promised on Monday to bolster military spending, ending years of cutbacks that had prompted critics to claim that Britain was retreating from its global role.

Announcing the outcome of a defense review, Mr. Cameron told lawmakers that his government would add 12 billion pounds, or about $18 billion, to the defense budget over the next decade, largely for military equipment, adding that such an investment was “vital at a time when the threats to our country are growing.”

The announcement, made 10 days after the terror attacks in France, came on a day in which Mr. Cameron also paid a brief visit to Paris, and promised the French government use of a British air base in Cyprus for France’s air campaign in Syria.

At present, British bombing raids against the Islamic State are limited to Iraq because of a vote in Parliament in 2013, but on Thursday Mr. Cameron will make the case to extend British airstrikes into Syrian territory.