The Nave Andromeda left Lagos, Nigeria, on October 6 and had been expected to dock in Southampton, England, at 10.30am Sunday (local time), according to ship tracking website MarineTraffic.com. The tanker has been circling an area about 5 nautical miles southeast of Sandown on the Isle of Wight for the past few hours, tracking data shows.
Chris Parry, a retired rear admiral in the Royal Navy, told the BBC that based on the available information, he suspects the stowaways ignored the crew's directions as the ship neared port, and the crew retreated to a secure area of the ship known as "the citadel" to retain control of the vessel.
"My guess is, they've got some rogue elements running around the ship," Parry said. "They don't want to go into shallow waters until they've secured those people. That's a really prudent thing to do."
Bob Seely, who represents the Isle of Wight in Parliament, said the British government was likely to convene a meeting of its emergency committee to discuss its response. Trouble on the ship is of particular concern because of the tanker's cargo and because the vessel started out from West Africa, he said.
"I suspect, because of the nature of this, it will be treated as marine counter-terrorism," Seely told Sky News. "The number of people in the UK who do that are very limited, and the relevant units will be looking at options, no doubt, as to what we could be doing."
The Home Office, which is responsible for law enforcement and immigration in the UK, said it was aware of the incident.
The UK Chamber of Shipping said it was "in contact with the relevant authorities."
"We believe it is likely to be related to stowaways onboard but are awaiting more information," the trade association said in a statement.