Police are dealing with a security incident involving a small number of stowaways aboard an oil tanker off the Isle of Wight, Sky News understands.
Sky’s home affairs correspondent Mark White said: “We believe at least half a dozen stowaways were discovered at some point today.
“There are reports the crew are sheltering at certain points on the ship.”
Authorities are still trying to establish the exact circumstances of the incident, but local reports suggest it could be an attempted hijacking.
The Nave Andromeda was due to dock in Southampton at 10.30am today and a Coastguard helicopter is circling the ship.
A restriction zone of five nautical miles is now in place around the vessel.
Hampshire Police have also deployed their own helicopter over the Isle of Wight.
Local MP Bob Seely has told Sky News he expects the incident will be handled by marine counter-terrorism forces and that a COBR meeting has likely either been held or is due to be held.
The 228-metre long ship is carrying 42,000 tonnes of crude oil and left Nigeria on 6 October before heading for Southampton.
The vessel had made a number of zig-zags around the area just off the Isle of Wight within a couple of hours, according to Marine Traffic.
If crew members are indeed sheltering on the ship, they could be on the vessel’s bridge, according to Sky’s Mark White.
They would be able to lock the area around the bridge to make it secure.
“It is clearly a serious incident that is unfolding,” Mr White said.
“For the authorities, as well as ensuring this tanker comes into no harm, they want to make sure any other vessels around this oil tanker are safe and out of the way.
“But the top priority for the authorities is to make sure the crew are safe.”
He added that the route is well-known for people smuggling – with immigrants coming from countries such as Iran, Iraq, Nigeria and Somalia – but the investigation is still in very early stages.
A Hampshire police spokesman said: “We are aware and dealing with an ongoing incident onboard a vessel which is situated south of the Isle of Wight.
“We will provide you with further updates when we are in a position to do so.”
The Coastguard confirmed it is assisting police, and search and rescue helicopters from Lee on Solent and Lydd are attending the incident.
Home Secretary Priti Patel is being kept informed about the incident, according to Sky’s political correspondent Joe Pike.
In December 2018, British forces stormed an Italian cargo ship and regained control after stowaways threatened crew members as it sailed in the Thames Estuary.