Amid the stacks of shipping containers towering over Britain's busiest port, an unusual suspected-crime scene unfolded this week.

Investigators wearing gas masks gingerly opened a row of 20m-long steel boxes and – after testing for noxious fumes – began inspecting the rusty entrails of 1,800 tonnes of scrap metal to see if it was sent illegally to pollute the environment 7,000 miles away.

Nearly 90 containers, each weighing more than 30 tonnes, have arrived back in the bustling Suffolk dockyard of Felixstowe in the past fortnight. Their journey began last November when they left scrapyards in southern England for Indonesia labelled as "recyclable" material with a value of $500,000 (£318,000).

The shipments were part of a lucrative trade – about 10 million tonnes of waste metal flow out of Europe each year. But when the Indonesian authorities inspected the contents of the British containers, they did not like what they found. The cargo was declared hazardous, resealed and British authorities were ordered to arrange for its immediate return.

Four UK companies are now being investigated by the Environment Agency (EA) to see if they sent contaminated and potentially toxic waste to the Indonesian capital, Jakarta, in contravention of laws designed to combat a global epidemic of cross-border dumping.

It is part of a wider picture in which millions of tonnes of material are flowing from the developed world to Asia and Africa, allowing criminals to profit by flouting international rules and passing off illegally-exported waste as part of a legitimate global trade in recyclable commodities worth £160bn a year.

The criminal trade, estimated to be worth at least £300m worldwide, ranges from hundreds of thousands of broken computers and televisions – so-called "e-waste" – sent to west Africa to be stripped of their heavy metals in unsafe conditions, to domestic waste smuggled out of Britain under the guise of recyclable paper or plastic. Used car tyres form an increasingly lucrative illicit market.

Andy Higham, the former detective who heads the 34-strong EA national crime team set up in 2008, said: "The financial benefits from environmental crime are similar to those from smuggling Class-A drugs but the actual penalties are very much lower. That is why it is attractive to criminals. Our job is to prove that they will be detected and will pay a heavy price."

The team investigating the containers returned from Indonesia – the inquiry is appropriately named Operation Anvil – will pick through their contents to try and find evidence the waste metal, claimed by its exporters to have been legitimately-exportable scrap metal, was mixed with hazardous contaminants allegedly found by the Indonesian authorities.

It is not an enviable task. The air was thick with the cloying smell of rust and damp as The Independent this week witnessed the opening of some of the first dozen containers shipped back from Jakarta for an initial inspection. As investigators wearing respirators slowly opened each box with a hydraulic jack to prevent tonnes of metal crashing on to them, a chemical specialist inserted a probe to test for ammonia or other gases before the doors were fully extended.

Inside one container was the oxidised, mangled product of a breakers yard – 5cm-thick chunks of steel plate, car parts, girders and unidentifiable bits of heavy plant – mixed with lengths of rubber hose, wood, broken circuit boards, plastic film and other detritus which could release toxins if smelted down in an unmodified furnace.

According to photographs taken by customs officials in Jakarta and sent back to Britain, suspect hazardous substances were found in some of the containers, meaning Operation Anvil will have to conduct further tests to assess which, if any, of Britain's strict waste export regulations have been broken.

Jeff Warburton, a senior environmental crime officer, who like other members of the crime squad is a former police detective, said: "The legislation is quite clear – there should not be any contaminants in this material. It is the very early stages of this investigation but we do take these things extremely seriously. It is absolutely right and proper that, given what the Indonesian authorities say they found, we conduct a thorough inquiry."

Looking along the line of containers, each of which may eventually have to be emptied to collate evidence, Mr Warburton, a former detective inspector with Greater Manchester Police, added: "It is expensive and it is time consuming but it is something that needs to be done.

"If material is exported that puts toxins and poisonous fumes into the air of a country where the furnaces are not equipped to filter out that material then that poses an obvious risk to the environment and the people. It is our job to stop contaminated waste being exported or, if it has left the country and been sent back, trace those responsible."

The suspect scrap shipments are part of a cash-rich – and perfectly legal – trade in waste metal for melting and recycling which has grown massively in recent years to feed commodity-hungry markets such as China. According to industry estimates, recycled goods provide 40 per cent of global raw material requirements.

At the same time, organised crime has spotted an opportunity to make quick money by loading cargo containers with mislabelled or illegal waste to be dumped abroad. With some 700,000 containers passing through British ports each year, it is impossible to check more than a fraction of cargos for illegal shipments.

Mr Warburton said: "It is the same pattern we see time and again. There is a legitimate business in which profits are to be made. Criminality sees that profit and seeks to exploit it by breaking the law."

Human rights campaigners and conservation groups have highlighted the damaging effects of the illegal trade. They point to Ghana and Nigeria where children pick through mountains of electronic waste rich in heavy metals and burn plastic casings off copper wire for less than a dollar a day, generating a rich cocktail of air and water-borne carcinogens.

In Jakarta, the authorities have reacted with alarm, pointing to the British shipments, which were mixed with 30 containers of suspect scrap metal sent from the Netherlands, as the latest in a growing influx of waste from the developed world which is shortening the lifespan of Indonesia's own landfills and causing health problems.

Masnellyarti Hilman, a senior government official dealing with hazardous waste, said: "Due to our lack of awareness, they sometimes send the waste illegally and others have falsely claimed that the waste was basic materials."

The scrap metal shipments are reminiscent of events in 2009 when 2,000 tonnes of municipal waste from across Britain, including used adult nappies, bags of rotting meat and used underwear were shipped to Brazil, allegedly under the guise of recyclable plastic.

The discovery of the shipments in three Brazilian ports sparked a diplomatic incident, with the country threatening a complaint to the World Trade Organisation. The then president, Luiz Inácio Lula da Silva, accused Britain of using his country as "the world's rubbish bin". A long and complex investigation by the Environment Agency, which traced the waste to locations from Wiltshire to Lincolnshire, is only now coming to a conclusion.

Two directors of an east London waste paper company last month pleaded guilty at the Old Bailey to exporting waste illegally, while two Brazilian nationals, who are alleged to have arranged the shipment, are due to go on trial in October.

These "reactive" investigations sit alongside the agency's preferred method of crime fighting, using intelligence to target suspected illegal exporters and other types of environmental criminals before they can do real damage.

As well as preventing the export of 51 containers of suspected illegal waste last year, this month the agency won an order requiring a crime boss jailed for four years earlier this year to pay back more than £900,000 earned from running a massive illegal waste site in Berkshire.

Mr Higham said: "Environmental crime is an extremely serious issue and we are beginning to get that message out. We know material is being stopped but the invisible side of the equation is what we have not stopped. I am not so arrogant as to say we are completely on top of this. But we are out there and we have the expertise and knowledge to catch those breaking the law."

On the scrapheap: Valuable rubbish

E-waste

Four million tonnes of waste computers and appliances are generated in the West each year. Working machines can be legitimately exported but millions of broken items, containing valuable but dangerous heavy metals, are exported illegally to Africa and Asia.

Scrap metal

There is a massive market in waste steel and metals. Annual exports from the EU grew by more than a third in the past decade to more than 10 million tonnes. Regulations require scrap metal to be free from contaminants but there is evidence countries accept dirty shipments.

Tyres

Used tyres can no longer be sent to landfill in the UK. But some operators are charging fitters to dispose of tyres and then illegally exporting them to other countries such as Vietnam.

Plastic

One of the biggest legitimate waste markets is in recyclable plastic. Last year, Britain exported 14 million tonnes of recyclable materials. Criminals exploit this demand by mislabelling domestic rubbish as packaging.

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