The prospect of dangerous instability on the Korean peninsula grew after South Korea vowed “resolute countermeasures” against its neighbour for the unprovoked attack.

The Cheonan, a 300 ft-long corvette, sank off the southern coast of Baengnyeong Island on March 26, within South Korean waters. Of the 104 men on board, 58 were rescued.

An official report found that a North Korean vessel had fired upon the Cheonan in the night before retreating back into North Korean waters.

The report was greeted with outrage by the US, Britain, Japan and the UN.

White House spokesman Robert Gibbs said: “The United States strongly condemns the act of aggression that led to their deaths.

“This act of aggression is one more instance of North Korea’s unacceptable behavior and defiance of international law…. Such unacceptable behavior only deepens North Korea’s isolation.”

William Hague, the British foreign secretary, condemned what he called North Korea’s “total indifference to human life”.

Britain “and international partners are committed to working closely with the Republic of Korea (South Korea) as they consider an appropriate multilateral response to this callous act,” he added.

UN chief Ban Ki-moon, who is South Korean himself, described the investigation results as “deeply troubling”, while the Japanese Prime Minister Yukio Hatoyama strongly condemned the attack on the Cheonan, describing it as “unforgivable”.

The report itself read: “The evidence points overwhelmingly to the conclusion that the torpedo was fired by a North Korean submarine..

“There is no other plausible explanation.”

The attack on the Cheonan is the worst apparent provocation by the North since the bombing of a Korean Air flight in 1987 with the loss of 115 lives.

The investigators unveiled large parts of the torpedo that had been salvaged from the scene, including its propellers, propulsion motor and steering section. They said these parts, some of which were inscribed with Korean lettering, “perfectly match the schematics of the CHT-02D torpedo included in introductory brochures provided to foreign countries by North Korea for export purposes”.

The South Korean Defence ministry also noted that a “few small submarines and a mother ship supporting them left a North Korean naval base in the West Sea two to three days prior to the attack and returned to port two to three days after the attack.”

North Korea has reacted angrily to the accusation, saying that the report was a “fabrication” and that it would wage “all-out war” if there was even a minor retaliation. North Korea has a history of sabre-rattling, but is also a nuclear-armed state, having tested an atomic bomb last year.

China, which is North Korea’s strongest ally, and which could use its veto at the United Nations Security Council to block any further sanctions against Pyongyang, reacted cautiously to the news.

The sinking of the South Korean ship was “unfortunate”, said Cui Tiankai, the deputy Foreign minister, without acknowledging that North Korea may be responsible for the incident.

Kim Jong-il, the North Korean leader, visited Beijing earlier this month, perhaps in an attempt to gauge whether the Chinese would continue to support him in the wake of the aggression.

“China is not directly involved, so it should not take a stance on either side or express views on the incident,” said Zhang Liangui, a North Korean expert at the Central Party School, where Communist Party leaders are trained.

“South Korea’s submission of its report to the UN will clearly force China into making a stance and this will be a challenge. This will be handled by the Foreign ministry, but my view is that China, in accordance with its rising status as a major country, should not go against the rest of the world, but should consider its interests in line with the majority,” he added.

In Seoul, the long weeks of mourning since the attack and the personal stories of the young men who lost their lives have deepened the sense of outrage, piling pressure on the government not to allow the lost lives to pass unavenged.

However, military retaliation against North Korea seems to have already been ruled out. “Nobody wants a war on the Korean peninsula and the truth is that it is not easy to take revenge after the event,” said Choi Jong-min, whose brother-in-law, Petty Officer First Class Jo Jin-young, was among the dead.

“Military reprisals should have been taken there and then [at the time of the sinking], or not at all,” he added.

South Korea has called an emergency meeting of its National Security Council on Friday to discuss its options. However, experts said that most of the punitive actions on offer stand to hurt Seoul at least as much as Pyongyang.

“There really are few good options out there for South Korea,” said Daniel Pinkston, a North Korea expert at the International Crisis Group. “They can go to the UN, but in reality China is very unlikely to back serious economic measures against the North which is already in economic crisis.

“Anything too drastic, such as military retaliation or real moves to destabilize the North’s economy risks regional instability that could trigger market crashes, capital flight and an overnight loss of regional confidence. It is really hard to see how the South ends up better off after this.”