British police said on Sunday they were dealing with an Incident on board a tanker off the Isle of Wight on England’s south coast, amid reports that stowaways may have seized control of the ship.

Police helicopters were seen flying over the Nave Andromeda, sitting around 5km off the coast of the Isle of Wight, with some media reporting that the vessel, owned by Greece’s Navios Maritime Acquisition, had earlier been denied entry to the Port of Southampton.

Vessel-tracking services showed the Andromeda had made unusual zigzags off the coast of the Isle of Wight.

However, John Thompson, co-founder of Ambrey, a maritime security firm that was providing advice to the vessel’s Greek owners on Sunday, said the incident was “definitely not a hijacking”.

“It’s stowaways who have got a bit rowdy with the crew,” Mr Thompson told the FT, adding that his company had stood down its anti-hijacking team.

Mr Thompson said his company had helped to put Navios Tanker Management, which manages the vessel on behalf of the owners, in touch with the correct authorities in the UK, such as border patrol, after the incident with the stowaways flared up.

While piracy in the Gulf of Aden and off west Africa has made tussles over the control of vessels common in some of the world’s busiest shipping lanes, security incidents are extremely rare on board vessels in UK waters.

The Andromeda, capable of carrying 500,000 barrels of crude oil or refined products, last called at the Port of Lagos, in Nigeria, on October 4, according to vessel-tracking services.

The vessel’s tracking information indicated it was sitting high out of the water, suggesting it was empty or nearly empty. ExxonMobil, operator of the Fawley oil refinery near Southampton, said the vessel had been due to call there on Monday to collect a cargo of gasoline.

Hampshire Constabulary, the police force for Hampshire and the Isle of Wight, said they were “aware of and dealing with” an “ongoing incident” on a vessel situated south of the Isle of Wight.

The Maritime and Coastguard Agency said it was assisting Hampshire Constabulary with the incident.

“The search-and-rescue helicopters from Lee-on-Solent and Lydd are in attendance,” the agency said.

Stowaways are a widespread problem in international shipping, with would-be migrants often able to board ships and hide amid the chaos of some big developing-world ports.