Chris Packham, the naturalist and TV presenter, has accused the government of sending “shivers of fear” through Britain’s environmentalists by backtracking on green pledges since Brexit.

The wildlife expert accused the government of “irresponsible and embarrassing” practices on plastic waste, following a report by the Guardian last week that the UK would continue to ship unsorted plastic waste to developing countries, even though the EU has banned the practice since 1 January.

Packham also criticised the recent UK approval for emergency use of a pesticide believed to kill bees, despite an EU-wide ban on its use outdoors and a British government pledge to keep the restrictions.

“These two things sent shivers of fear and shockwaves through the environmental community,” said Packham. “It looks like we will have a toxic fight on our hands.”

Packham was one of several environmentalists, experts and MPs – from Labour, the Liberal Democrats and the SNP – to sign an open letter to the government on Thursday, accusing it of setting a “dangerous precedent” by failing to maintain its environmental promises.

The letter, backed by groups campaigning to reduce plastic waste, urged an end to “post-Brexit backtracking” on environmental protections and called for an immediate ban on the export of unsorted plastic waste to developing countries.

It read: “Leaving the EU was touted as Britain’s opportunity to set itself apart on the world stage as an environmental leader. This was supposed to be a ‘Green Brexit’, but the actions of the government thus far suggest otherwise.”

Signatories included Georgia Elliott-Smith, Unesco special junior envoy for youth and the environment, Prof Sir Brian Hoskins of Imperial College London and Dr Paul Butler of the University of Exeter, as well as several MPs.

Earlier this week a petition set up by a nine-year-old girl after she read the Guardian report, calling for a ban on exports of unsorted plastics to non-OECD countries, attracted more than 70,000 signatures.

Packham said: “EU legislation on the environment has been quite robust, on things like the birds directive and the habitats directive. But in the first few days of January, we have seen our government ship our problematic unsorted plastic waste around the world to countries who don’t have the capacity to deal with it.”

He compared the practice to someone from a “posh neighbourhood chucking your rubbish into the garden of someone from the not-so-well-off neighbourhood next door and forgetting about it. Frankly, it’s irresponsible and embarrassing when we have the capacity and technology to deal with it, if only we put in the resources”.

New UK regulations say that in order to comply with international rules to tackle transboundary plastic waste, which began in January, any unsorted or difficult-to-recycle plastic waste can be sent to non-OECD countries, providing there is prior informed consent by the exporter and importer. The new rules come despite a Conservative party manifesto commitment to banning the practice, and promises of no regression on environmental standards following Brexit.

Citing the climate and the biodiversity crises that have focused the minds of the scientific, environment and conservation community, Packham said: “We have an enormous capacity going forward to help the environment. Instead, we have the burden of fighting the government not to back out of EU legislation to protect the environment.”

The UK is the second biggest per-capita producer of plastic waste in the world. Around two-thirds of its plastic waste is sent abroad.

The Environment Agency estimates that annually about 210,000 tonnes of plastic waste is sent from England to non-OECD countries including Malaysia, Indonesia and Pakistan.

Many of these countries do not have an adequate infrastructure to process the waste, which is burned or dumped. Eventually much ends up in the ocean.

Sian Sutherland, co-founder of international campaign group A Plastic Planet, said: “If the government continues to backtrack on key environmental pledges we are never going to see the ‘Green Brexit’ we were all promised.

“With Cop26 right around the corner, now is the time for Britain to show the world it means business when it comes to tackling the pressing issues of plastic pollution and climate change, but we will never truly combat the plastic crisis if we continue to hide our guilt in other people’s backyards.”

Rebecca Pow, environment minister, insisted the government is not backtracking on its commitments to ban plastic waste exports to non-OECD countries but she did not provide a timetable for when the ban might be implemented.

Pow said: “The UK government is a global leader in tackling plastic pollution and we are absolutely not backtracking on our commitments in this area. Unlike the EU ban, our manifesto commitment to ban plastic waste exports to non-OECD countries is not limited to just one category of plastic waste.”

The department said it had commissioned research to better understand existing UK plastic waste recycling capacity and would consult in due course on how to deliver its manifesto commitments.