A highly toxic weedkiller not authorised for use in the EU is being exported to developing countries from a UK factory.

Paraquat, a pesticide so lethal that a single sip can be fatal, has caused thousands of accidental deaths and suicides globally, and was outlawed by EU states in 2007.

But Swiss pesticide manufacturer Syngenta is exporting thousands of tonnes of the substance to other parts of the world from an industrial plant in Huddersfield.

Campaigners have condemned the practice as an “astonishing double standard”, while a UN expert said it was deeply disquieting that the human rights implications of producing a substance for export that is not authorised in the EU were being ignored.

“The fact that the EU has decided to ban the pesticide for health and environmental reasons, but they still export it to countries with far weaker regulation and far weaker controls, is shocking to me,” said Baskut Tuncak, the UN special rapporteur on toxic wastes.

Syngenta is responsible for 95% of Europe’s exports of paraquat, which it sells under the brand name Gramoxone. The substance can be absorbed through the skin and has been linked with Parkinson’s disease.

Syngenta has exported 122,831 tonnes of paraquat from the UK since 2015, an average of 41,000 tonnes a year, according to export licensing data analysed by the Swiss NGO Public Eye and shared with the Guardian.

Since 2015, when a facility in Belgium stopped exporting paraquat, all EU exports of the pesticide have come from Syngenta’s UK base, according to Public Eye.

Almost two-thirds of these exports by volume – 62% – go to poor countries, including Brazil, Mexico, Indonesia, Guatemala, Venezuela and India. A further 35% is exported to the US, where paraquat can only be applied by licensed users.

Syngenta has been manufacturing paraquat at its Huddersfield plant for two decades. In 2014, the company was fined £200,000 after more than three tonnes of the chemical was spilled in an industrial accident.

A Syngenta spokesman described the Huddersfield spill as a source of regret, but pointed out that nobody had been injured and said there had been no risk to the environment or public.

“Paraquat has for half a century been among the world’s most effective and environmentally beneficial herbicides,” said the spokesman. “It has helped millions of farmers stay productive and competitive.”

He added that the herbicide is authorised for use in countries with “some of the most demanding regulatory systems”, including the US, Australia and Japan.

But Tuncak called for tighter controls on the pesticide, which he said is most widely used in developing countries.

“The impact is often felt outside the countries where companies implicated are headquartered,” Tuncak said. “It’s a double standard and an example of the need for global controls.

“While the UK has played a leading role in tackling modern slavery, it has turned a blind eye to the human rights impacts of UK business regarding their toxic supply and value chains.”

Laurent Gaberell of Public Eye said: “Paraquat causes thousands of poisonings every year in developing countries. The UK bears an enormous responsibility … To subject individuals of other countries to pesticides known to cause major health damage or fatality is a clear human rights violation.”

Nick Mole of the Pesticide Action Network described the practice of exporting chemicals deemed too dangerous to be used within the EU as an “astonishing double standard”.

The EU is not the only region vulnerable to such accusations: China, the world’s largest producer, is in the process of phasing out its use, Mole said.

“There’s no reason for it to carry on whatsoever,” he added. “It’s a massive failing in the regulatory system: if a pesticide is banned, it shouldn’t be exported for use in other countries.”

The industry argues that paraquat is among the most effective herbicides, has few alternatives, and is safe providing it is used properly.

But Mole said: “There are plenty of real alternatives – otherwise we wouldn’t have phased it out in the EU. People can do without it, but it might dent the profits of Syngenta, and they will fight tooth and nail for those profits.”

The Syngenta spokesman argued that paraquat was not banned by the EU, but said its registration for use in the bloc was annulled by a 2007 European court of justice ruling. “Neither the decision, nor the commission’s interpretation of it, constituted a ban,” he said. “The court was not qualified or empowered to – and did not – reach any conclusion that paraquat was inherently unsafe or dangerous.”

The ruling found that a 2003 decision to permit the use of paraquat did not “satisfy the requirement of protection of human and animal health”.

A spokeswoman for the Department of Farming and Rural Affairs said the UK is a signatory to the Rotterdam convention, which regulates transfers and labelling of pesticides and other hazardous chemicals.

“The UK supports global action on hazardous substances which meet the scientific criteria for listing under the Rotterdam convention such as paraquat dichloride,” she said.

Other countries are understood to have blocked the proposal.