The Talavera changed its name to Chief Ahmed in November around the time a Hamas military commander called Ahmed Al-Jaabari was assassinated by Israel.

In this case tracking systems showed Pioneer undertaking two parallel journeys in late October thousands of miles apart.

On one track, the tanker appeared to sail from the Suez Canal to the Red Sea - stopping off in Jordan and Yemen on its way to Iran - while at the same time travelling through the South China Sea to the Chinese port of Ningbo.

“Using another MMSI other than your own can only be done among the same flag members and has to be done by one of the workers in the flag offices,” said a Western diplomatic source, who monitors efforts to track Iranian tankers.

All ships registered in mainland Tanzania or Zanzibar fly the Tanzanian flag, and officials in both offices said they were unaware of any Iranian vessels on their register.

Responding to diplomatic pressure by the United States and European Union to drop all Iranian tankers from their registries, Tanzania’s foreign minister issued a statement denying Iran’s vessels had been legitimately registered.

“All the 36 Iranian ships were de-registered and hence stopped using our national flag. We have not registered any new ships as claimed,” said Bernard Membe, adding that Tanzania had asked the US and EU to help investigate the Dubai-based agency that had registered the vessels.

“If we establish that this (Iranian tankers have been registered) has happened we will cancel the registrations.”

Vessels without a flag cannot be insured, dock in most ports or use the vital Suez Canal shortcut between the Mediterranean and Indian Ocean.

Manipulating or turning off satellite tracking devices is not necessarily an indication that a vessel is trying to conceal illegal activity, according to International Maritime Bureau director Pottengal Mukundan.