Tam Chi-man, 38, who suffered 13 per cent burns, was in a critical condition, while his two colleagues, aged 27 and 48, were in a serious condition with 10 per cent burns.

The officers were investigating a noise at the back of the nine-metre vessel at 6am in Sai Kung typhoon shelter, Marine Police East divisional commander Superintendent Chan Lik-kin said.

'When one of the officers tried to remove the generator's plastic cover, there was an explosion. The cover broke open and the generator burst into flames,' he said.

'The three officers sustained serious burns, mainly to their faces, hands and legs, in the blast. They were brave to fight the fire with extinguishers before fireboats arrived.'

The blaze was put out before the fireboats and other marine police launches arrived.

Police said the Seaspray vessel was being used as a reporting centre and resting place. It was anchored off Sai Kung on Monday night when the three constables went on patrol in an inflatable boat. 'Preliminary investigations show that an unknown inflammable gas accumulated inside the plastic cover of the generator and a spark apparently caused by a loose wire triggered the explosion,' Mr Chan said.

'Further tests are to be conducted to find out the exact cause of the incident.

'Such an explosion had never happened since the boat came into operation in 1992. After the incident, we ordered a check of the other craft of the same model and no problems were detected.'

The fibreglass and metal vessel was towed to a government dockyard for examination.

Mr Chan said in addition to the police investigation, staff from the Electrical and Mechanical Services Department would inspect the vessel and submit a report.