Sri Lankan authorities on Wednesday expelled an Antigua-registered ship that entered the island's territory without declaring a radioactive cargo bound for China.

The country's Atomic Energy Regulatory Council said the MV BBC Naples was asked to leave after it was found to be in the Chinese-run port of Hambantota on Tuesday night carrying uranium hexafluoride.

"The ship failed to declare its dangerous cargo -- uranium hexafluoride -- and we decided to order it to leave our waters immediately," council director general Anil Ranjith told AFP.

The ship had come from Rotterdam but authorities did not say where in China it was headed.

Ranjith said it was an offence to enter a port without declaring the material, which is used to enrich uranium, the fuel for nuclear power stations and weapons.

Sri Lanka's opposition leader Sajith Premadasa demanded an investigation into the incident, describing it as a serious safety threat.

"The navy has not been allowed to board the vessel to carry out an inspection," Premadasa said.

There was no immediate comment from the government.

Hambantota port was leased to China in 2017 for 99 years after the Colombo government was unable to repay $1.4 million it had borrowed from Beijing to build it.

Hambantota, about 260 kilometres (162 miles) south of Colombo, is near key Indian Ocean shipping lanes.

The entry of two Chinese submarines into Colombo in 2014 angered neighbouring India, the traditional regional power which is competing with Beijing for influence in the Indian Ocean.

Since then, Sri Lanka has not allowed Chinese submarine visits.