Tanzania has halted the registration of overseas vessels following a spate of drugs seizures.
President John Magufuli ordered a probe into the 470 ships flagged in the country to ascertain their activities.
He said the investigation was due to "drug seizures on board Tanzanian-flagged vessels," a statement from the presidency said.
News24 reported the president as saying: "We cannot allow the name of our country to be tarnished by individuals pursuing their selfish interests."
Magufuli withdrew Tanzania's flag from two vessels seized over drugs in recent months.
The Paris MOU on port state control rates the flag as high risk.
In 2012, the Tanzania Zanzibar International Register of Shipping (TZIRS) kicked 36 Iranian-owned vessels off its list after they allegedly tried to evade sanctions.
In December, Dutch naval forces intercepted a Tanzanian-flagged cargoship carrying 1.6 tonnes of cocaine.
And another Tanzanian vessel was held in Crete this month with an explosives cargo.
The master said he had been ordered to head to Libya, but the owner denied this was the case.