The UK is planning to launch a formal investigation after the discovery in Colombo of hazardous clinical waste, including suspected human remains, thought to have been exported to Sri Lanka.

Officials at the port of Colombo discovered the rotting waste after a foul smell emerged from more than 100 containers, which had been imported under the cover of metal recycling. Sri Lanka’s environment agency has ordered the return of the waste.

It is common for wealthy countries such as the UK to pay to send their recyclable waste overseas to developing countries; the majority of these imports end up rotting in landfills or being illegally incinerated.

“We know it is clinical waste and we also suspect the containers to contain human remains as well due to the odour,” said Sunil Jayaratne, a Sri Lanka customs spokesperson. “It is very difficult to examine due to the odour and the fluids that are coming out of the containers.”

A spokesperson for the UK’s environment agency said the government was in touch with Sri Lankan officials: “We are in contact with the Sri Lankan authorities and have requested more information, which would allow us to launch a formal investigation.”

Environmentalists have expressed serious concerns over the containers, which they say pose a major risk to public health and have been stuck in Sri Lanka for up to two years.

In total, 241 containers were brought to Sri Lanka and 130 of these were taken to a free-trade zone for recycling and re-export, according to Sri Lankan customs. The imports contained a mixture of mattresses, plastics and hospital waste.

Suranjan Kodithuwakku, the chairman of the Green Movement of Sri Lanka, which represents more than 150 environmental groups, said there is a risk that groundwater and surface water may be contaminated.

The import of waste is a growing concern, added Kodithuwakku, who said materials were often burned in container yards in Colombo or taken to dumping grounds in populated areas. “It has been recorded continuously that [there are cases] of water-borne diseases, skin diseases and respiratory diseases because of these garbage dumps,” he said. “These kinds of things still aren’t properly managed by the authorities.”

Jayaratne said a team had been appointed to investigate the containers. “We have to send it back to the country of export very soon. We have to take action against the culprits also. We will first send the goods back and then take the action. According to the Basel convention, the country of export is responsible for the goods.”

The Basel convention was passed in 1989, following concerns about the dumping of hazardous waste in poorer countries. In May, an amendment to the convention stated that nations have the power to block the import of contaminated or hard-to-recycle plastic rubbish.

“In the initial video that they posted on the news you could see this weird liquid – a greenish, blueish liquid – coming out of the containers and into the waterways,” said Avishka Sendanayake, a consultant on climate change and sustainability. “We don’t know what kind of chemical waste this is,” she said.

Sendanayake took part in small protests outside the British high commission in Colombo on Wednesday, demanding the UK to take back its waste. “We are still struggling to deal with waste within the country; there is no way we can deal with the waste coming in from another country,” she said.

In 2017, 19 people were killed, including five children, after a huge rubbish dump collapsed on to their homes in Colombo.

Last year, China banned foreign plastic waste imports, causing trash to flow instead to Sri Lanka and across south-east Asia. Countries have quickly become overwhelmed and are increasingly taking a stand over the dumping of waste by foreign nations.

Earlier this month, Cambodia announced it would send 1,600 tonnes of plastic waste found in shipping containers back to the US and Canada, while Indonesia said it would send back dozens of containers full of waste to France and Australia. Earlier this year, Malaysia said it would ship 450 tonnes of imported plastic waste back to its sources.

A UK Environment Agency spokesperson said: “We are committed to tackling illegal waste exports, which is why individuals found to be exporting incorrectly described waste can face a two-year jail term or an unlimited fine.”