Floating armouries - ships packed to the gunnels with machineguns, ammunition and other military equipment - are operating in international waters with a "worrying lack" of regulation, warns a report that says the vessels could pose a threat to regional peace and stability.
The armouries were set up to supply private security guards employed to protect shipping from pirates, particularly off the East African coast. The report, commissioned by the Remote Control Project, a body that raises awareness of new military trends, said there was an "urgent need" for an international agreement to set minimum standards.
It also said there should be frequent safety checks and firms should not be allowed to use flags of convenience issued by nations blacklisted for low maritime standards. The number of floating armouries is unknown as no international register exists.
The report, written by the Omega Research Foundation think-tank, lists 33 vessels, but there could be more. Eight sail under the flag of landlocked and blacklisted Mongolia.
The report noted the United Nations Monitoring Group on Somalia and Eritrea had "raised concerns that ... floating armouries and private maritime security companies could represent a threat to regional peace and stability.
"None of the vessels used as floating armouries has been purpose-built as an armoury; instead, they are adapted craft. As a result, vessels may not have safe and secure storage for arms and ammunition," the report said.
Andrew Smith, a spokesman for the UK-based organisation Campaign Against Arms Trade, said: "Unfortunately, it is characteristic of an illegitimate and immoral arms industry that receives very little scrutiny and oversight ... Putting more weapons into unstable environments can bring unforeseen and deadly consequences."
MNG Maritime, a London-based company, has a floating armoury based off Fujairah in the Gulf of Oman and another due to be deployed in the Red Sea next year.
Mark Gray, MNG's director, said he would welcome more transparency in the industry, and backed the idea of a new international safety standard. "Overall, the [Omega] report was completely fair. Companies must stop exploiting countries with 'black-classification', which means they can get away with the bar set very, very low for safety," he said.
"It would be very sensible to put this power in the hands of the International Maritime Organisation."