Prime Minister Keir Starmer of Britain promised on Monday to bring his country to “war-fighting readiness” as he announced plans to build up to 12 new attack submarines and invest billions of pounds in nuclear and other weaponry as part of new military strategy for a more dangerous world.

“The threat we now face is more serious, more immediate and more unpredictable than at any time since the Cold War,” Mr. Starmer said at a media conference on Monday morning, pointing to “war in Europe, new nuclear risks, daily cyberattacks,” and “growing Russian aggression,” in British waters and skies.

“I believe that the best way to deter conflict is to prepare for it,” Mr. Starmer added, speaking at an industrial facility in Glasgow, ahead of the release of the government’s strategic defense review, which will outline plans to increase production of drones and increase stockpiles of munitions and equipment.

Britain’s ambitious rearmament plan comes against the gathering clouds of Russia’s attacks on Ukraine, American disengagement from Europe and rising global tensions. Mr. Starmer presented his plans hours after one of the most intense aerial bombardments of the three-year war, with Ukrainian drones striking air bases deep in Russian territory.

The review, led by George Robertson, a former secretary general of NATO, was set up last year soon after Mr. Starmer won a general election. But its task was given fresh urgency amid growing evidence of President Trump’s weakened commitment to European security and his ambivalent and at times ingratiating attitude toward President Vladimir V. Putin of Russia.

The review is scheduled for release on Monday afternoon but among the recommendations made public in advance by the government were the procurement of up to 7,000 British-built long-range weapons and the creation of a new cybercommand, alongside an investment of a billion pounds, equivalent to $1.35 billion, in digital capability.