Police and coastguard officers are dealing with reports of an attempted hijacking of a Liberian-registered tanker off the Isle of Wight.
The vessel, the Nave Andromeda crude oil tanker, was said to have been forced to anchor south of Sandown.
A Hampshire police spokesman said: “We are aware and dealing with an ongoing incident on board a vessel which is situated south of the Isle of Wight.”
The Ministry of Defence denied it was involved in the incident or that any members of the Special Boat Service had been scrambled.
Police and coastguard staff were handling the operation, the MoD said.
A spokeswoman for Associated British Ports, which runs Southampton port, said it had not had any contact with the vessel and had no further comment.
It is understood that the 228-metre tanker had been expected to dock in Southampton at 10.30am on Sunday.
Two coastguard helicopters have been scrambled to the scene and were spotted circling the ship as it adopted an erratic, zig-zag course through the Channel. An exclusion zone was set up south of Sandown. The vessel was reported to have left Lagos, Nigeria earlier this month.
An aerial photograph of the ship, shared on Twitter, showed it sitting high in the water.
Unconfirmed reports suggested that a number of stowaways may have attempted to take control of the ship as it approached the UK.