'The Lisbon Maru was sunk very shortly after a number of major British disasters in Asia,' says Banham. 'The fall of Singapore was such a big shock to Winston Churchill that it overwhelmed the other seemingly smaller tragedies around it. The Lisbon Maru's fate was hidden by that bigger tragedy.'

Banham says he wrote The Sinking of the Lisbon Maru, Britain's Forgotten Wartime Tragedy as a memorial to those onboard, who had been part of the Hong Kong Garrison and included Royal Navy, Royal Scots and the Middlesex Regiment among other military personnel. Most were British, with a few Australians and New Zealanders.

As in his previous book, Banham gives a chronological account based on historical documents and oral histories.

After the ship left Hong Kong it hugged the China coast, its captain aware of the danger of enemy submarines. When the crew of the USS Grouper saw the Lisbon Maru they identified it as Japanese.

The survivors recount how they heard the first torpedo pass under the boat, so when the ship was struck, about 7am, it came as no surprise. The Japanese soldiers battened down the hatches on the three holds because, they told a war crimes trial, they feared that navy personnel among the prisoners would try to take the ship.

The book recounts the harrowing tale of how, during the next 24 hours, exhausted and malnourished prisoners, many suffering from dysentery and beriberi, tried to pump water out of one flooding hold where they were held in the dark without food or water. They took turns lying down. The stench was overwhelming.

The prisoners eventually tried to escape, hundreds succeeding under fire from Japanese soldiers on deck. Survivors jumped into the sea where they were shot at or run down by Japanese escort boats.

Some survivors Banham spoke to hadn't talked about that terrible night for 60 years. Some found it cathartic to recount the experience. 'It depends on who you talk to and what age the POWs were at the end of the war,' he says.

'Many really didn't want to look back on those years at all. They wanted to start afresh. And, of all the groups of POWs I've spoken to, the group where this was most obvious was the Lisbon Maru survivors because of the vast trauma they had been through. They wanted to put the past behind them, but then at a certain age, they wanted to go back over those memories.'

Most just wanted to get on with their lives until about five years ago, when their attitudes changed. 'They felt that the whole incident might be forgotten, and that seemed to matter to them more and more,' says Banham. 'That made them extremely helpful and co-operative.'

Only a handful drowned in one of the three holds when the ship went down. A ladder on the side of the hold had broken, leaving no way out. Many died in the sea - shot, drowned or swept away. Chinese fishermen from Zhoushan island, off Shanghai, helped fish the prisoners out of the water in front of the Japanese boats. Banham recently visited three of those fishermen to hear their stories. About 400 prisoners were saved on Zhoushan. Eight hundred died that day, and a further 200 succumbed to shock, exhaustion and illness in the weeks that followed. Of the 4,500 of Hong Kong's garrison who perished during the entire war, 1,000 died directly or indirectly from the sinking of the Lisbon Maru.

For those who returned to Britain, New Zealand and Australia, there were no counsellors to help them cope with post-traumatic stress. 'They were told to just forget about it,' says Banham.

Several of them recently visited Hong Kong, including Jack Etiemble, 84, who joined the army as a boy wanting to travel, and became a POW in Hong Kong a few days before his 19th birthday.

Decades later in Australia, he was to meet Garfield Kvalheim, who had been a submariner aboard the USS Grouper. While no blame has been laid by the survivors - there was no way of knowing the Lisbon Maru was carrying 2,000 prisoners - Kvalheim wanted to contact those willing to meet him. 'Jack introduced [Kvalheim] to some of the other survivors and they had some very successful and enjoyable reunions,' says Banham.

'The POWs had never blamed the Americans. They knew that the Americans thought it was just a normal Japanese freighter. There was no way the Americans could have concluded anything else. I think it was the Americans who worried about it far more than the POWs. And it was probably a matter of the survivors consoling the submariners rather than the other way around.'

Banham avoids emotive language. It's the story of a tragic incident, he says, and needs no embellishment. He's planning four more books on Hong Kong's involvement in the second world war, but wanted to finish the story of the Lisbon Maru while some survivors remain.

The Sinking of the Lisbon Maru, Britain's Forgotten Wartime Tragedy, by Tony Banham (Hong Kong University Press, $250)

Tony Banham will discuss his latest book, May 30, 7.30pm, FCC, 2 Lower Albert Rd, Central, $175 (members) $250 (guests). Inquiries: 2521 1511