The British are engaged in a searching inquiry into the future of their nuclear deterrent, including the possibility of disbanding it, that illuminates a Japanese debate over whether to acquire nuclear arms.

The British deterrent, which comprises four submarines carrying ballistic missiles, will reach the end of its service life in 2024. Because it will take about 14 years to replace that force, parliamentary decisions must be made by 2010.

The Japanese, confronted with North Korean missiles and a Chinese nuclear arsenal, have begun to consider whether they should acquire nuclear arms. Prime Minister Shinzo Abe has asserted that the question is closed, but some Japanese contend that the issue should be debated and a consensus reached, which is the way many critical decisions are made in Japan.

In this, Britain and Japan are similar: Both are tight island nations off the coast of the Eurasian land mass, are parliamentary democracies, allies of the United States, and have roughly comparable armed forces.

Neither has the space to deploy land-based missiles or aircraft that could survive enemy assaults. Britain has thus put its deterrent to sea and that would be the only viable alternative for a nuclear-armed Japan. Based on British experience, a Japanese deterrent would be expensive, up to a total of $145 billion over 30 years for a nation whose annual defense budget is $45 billion.

By any reasonable measure, the obstacles to Japan becoming a nuclear power are immense.

Japan still has a pervasive nuclear allergy, the legacy of the U.S. atomic bombings of Hiroshima and Nagasaki to end World War II in 1945. That would cause a profound division preventing a consensus on nuclear weapons.

A decision to go nuclear would almost certainly require that Article IX, the "no war" provision in the constitution, be amended. By definition, the decision would destroy the three non-nuclear principles adopted in 1967 to preclude Japan from possessing, producing or hosting nuclear arms.

China would protest vigorously. South and North Korea would condemn Japan and perhaps accelerate moves to reunify, with continued hostility toward Japan. The U.S. would object because a nuclear-armed Japan would further weaken the nuclear non-proliferation regime.

Moreover, Japan would be obliged to consider various international obligations, including the Nuclear Non-Proliferation Treaty, the Comprehensive Nuclear Test-Ban Treaty and the Missile Technology Control Regime.

Some Japanese strategists argue that Japan's most immediate threat is from North Korean missiles against which a small nuclear deterrent would be ineffective. The nation's funds, they contend, would be better spent on missile defenses.

Setting aside those considerations and looking only at the military aspects, Japan surely has the technology to produce nuclear warheads, given the experience of generating a third of the nation's electricity with nuclear power.

As a Japanese strategic thinker said some years ago, "Japan is N minus six months," meaning it would take only six months to build a nuclear device after a decision had been made. Japan would also need to invest in the means to hit targets to have a credible deterrent.

Because Japan lacks the continental space of the U.S., China or Russia in which to base bombers or missiles, the Japanese must look to submarines. A British report for Parliament last month noted that, during the Cold War, its submarine fleet was "mobile and difficult for the Soviets to track."

Today, Britain's force comprises four nuclear-powered Vanguard-class submarines, each carrying 16 missiles on which can be mounted three warheads apiece. The British built the submarines and warheads but bought from the U.S. the Trident missiles that have a range of 4,000 to 7,200 miles, depending on how many warheads a missile carried.

The report, titled "The Future of the British Nuclear Deterrent," says: "From the decision in 1980, it took 14 years to complete the acquisition of the Trident capability with the first Vanguard-class submarine entering service in December 1994." The fourth submarine, Vengeance, went to sea in 2001.

The overall investment was estimated at $29 billion in 2006 prices. The report said the expenditure on a successor system would depend on the option chosen and could run up to $147.6 billion.

During the Cold War, the British had planned to have two submarines at sea ready to launch strikes. After the Cold War, only one submarine is on patrol, with another ready to sail and the other two in training or maintenance.

With this minimal force, Britain continues to rely on the U.S. for its overall nuclear deterrent. Japan could well decide to do the same.

To see the British nuclear report, visit: www.parliament.uk/commons/lib/research/rp2006/rp06-053.pdf.

Richard Halloran is a Honolulu-based journalist and former New York Times correspondent in Asia. His column appears weekly in Sunday's Focus section.