Two workmen were rushed to hospital after a gangway leading to the warship HMS Belfast collapsed, plunging into the Thames.
More than 100 people on board had to be evacuated in small boats following the incident at the World War Two tourist attraction which is moored on London's south bank between London Bridge and Tower Bridge.
The embankment near the ship was cordoned off with barriers to prevent people getting too close as an RNLI lifeboat from Tower Lifeboat Station scoured the river for any further casualties.
Emergency: Two men were rushed to hospital after the gangway leading to the tourist attraction HMS Belfast gave way
It appeared to have sheared off at the point where visitors would step onto the ship. The two men had suffered abrasions and were taken to St Thomas' Hospital as a precaution. A third escaped unhurt.
Speaking at the scene, HMS Belfast director Phil Reed said one of the three sections of the walkway had 'sheared' away from the rest.
He said: 'As far as we know, the brow, where it joins the ship, sheared off and fell into the water.
'The two contractors on the brow got the warning as things began to rattle and they ran off and sustained only minor injuries.'
Search: Emergency workers scour the surrounding water for any further casualties
Injuries: The two workmen suffered abrasions and were taken to St Thomas' Hospital as a precaution
Mr Reed described the incident as 'extremely unfortunate' and said he was 'flabbergasted' to have heard it had happened.
There were around 90 visitors on HMS Belfast at the time, including 30 schoolchildren who were staying on board as part of an experience called Kip on a Ship, he added.
A fire engine could be seen parked at the entrance to the walkway and two ambulance crew went back and forth between it and an ambulance parked outside the cordon.
A section of the Thames Path was shut to allow emergency vehicles to get close to the ship. Later a group of workmen in high visibility jackets were seen leaving the site.
HMS Belfast saw action in both the Second World War and the Korean War and is permanently docked on the Thames as part of the Imperial War Museum.
A spokeswoman for the museum said all on board the vessel were safely evacuated on boats after the gangway collapsed at 12.47pm.
Evacuation: Lifeboat crews and the river police keep watch as members of the public are helped onto a small boat
Twisted wreckage: A close-up of the damage to the metal walkway
A spokesman said that the ship will be closed until further notice.
He added that an 'urgent and full investigation' had been launched into the cause of the collapse on the ship.
He said: 'The gangway which leads visitors onto HMS Belfast has collapsed. Two contractors working on the site have sustained minor injuries and have been taken to hospital as a precaution.'
Historic: The Royal Navy cruiser was built in 1938 and served in both World War Two and the Korean War
'The Port of London Authority and emergency services have been informed.
'All visitors and the majority of staff members have been evacuated and escorted back to shore.
'A small group of staff members are still on HMS Belfast and a boat is being arranged to take them to shore later this afternoon. Our priority is to ensure that all our visitors and staff members are safe.
'The ship will be closed until further notice.
'We are beginning an urgent and full investigation into what has happened.'
Plans for an emergency training exercise on board the ship tomorrow afternoon have been cancelled.
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