Chris Packham has accused eBay of 'irresponsible cruelty' for allowing pet fish to be posted in 'fizzy drink' bottles, with hundreds dying in the post.
Shocking photos have emerged online of fish in a serious state of distress after being purchased from the online marketplace and sent in subpar packaging.
eBay's live animals policy says sellers can list aquarium or pond fish up to 5cm long but they need to have necessary government permits and guarantee safe overnight delivery.
But Packham said it was 'blatant persecution' and to suggest that there was a 5cm limit to 'suffering and cruelty was absurd'.
Pictured, fish which were allegedly posted in the mail after being purchased on eBay. Hundreds of fish are said to have died being posted in subpar packaging
Packham CBE (pictured) said it was 'blatant persecution' and to suggest that there was a 5cm limit to 'suffering and cruelty was absurd'
He added: 'We know that fish can experience stress and pain and to suggest this stops beneath certain dimensions is both illogical and irresponsible.
'eBay has a moral responsibility, there is an ethical duty to ensure their selling and shipping does not result in a literal 'last post' for these sentient beings.
'If people care about their companion animals, the living creatures they share their homes and gardens with, they need to take responsibility for where and how they purchase their pets.'
The animal welfare campaigner spoke out after customer Tara Smith claimed she snapped up 10 of the 'peaceful' cardinal tetra to add to her home aquarium set-up from a private seller on eBay last month.
Fish can be seen swimming around in a fizzy drink bottle looking pale and distressed following their journey
Tara Smith says she snapped up 10 of the 'peaceful' cardinal tetra to add to her home aquarium set-up from a private seller on eBay last month
When she opened her mail, she was stunned to discover one fish was dead already.
She also said the remaining usually vibrant red and blue fish were 'pale and stressed' while listlessly swimming around in a 330ml bottle.
Four others died days later after the journey that the disgusted shopper likened to them being 'thrown around in a washing machine'.
Ms Smith, from near Sheffield, South Yorkshire, said: 'It makes me angry and really upset to know people can transport any animal without any care or compassion for the animals' well-being.
'Some will say 'it's just fish' but even fish have a brain and can feel pain and suffering.
'I'm not sure how long they were in the pop bottle for or even how long they were in the postal system.
'They arrived pale due to stress and probably lack of oxygen. Imagine yourself in a tight space being thrown around like if you were in a washing machine.
'And on top of that the oxygen being slowly depleted.
'Sadly not all survived. Out of the 10, one had died on arrival, four more died days later so only five are now healthy and living a good life.'
eBay confirmed they allowed sellers to post live fish to customers providing they were under 5cm and could 'guarantee safe delivery'
Five of Tara Smith's fish died during the post. The surviving fish are said to be doing well
Ms Smith ordered 10 of the fish from a private seller on eBay, which landed on her doormat on June 22.
Posting the shocking pictures online, she said: 'Has this group got a 'don't buy from' list because what I have just received has shocked me.
'This seller wants banning from ever keeping fish, let alone selling them.
'Paper bag wrapped around a pop bottle and delivered by Royal Mail of all people. All fish safely in a hospital tank and hopefully recover.'
Horrified fish lovers commented on the post with one saying: 'OMG that's awful, are they actually for real? It's a wonder they're still alive.'
Another commented: 'I feel like you've done them a favour though, you've literally saved these little fishies due to your quick thinking, well done to you!'
One added: 'I think it has to be a private seller 100 per cent. No shop in the right mind would post fish like that, no human should ship fish like that.'
The RSPCA said they will be 'reaching out to eBay to discuss their current policies'.
eBay's website has many listings of live fish for sale including dozens of listings for the cardinal tetra that Ms Smith purchased, most with the option 'Post to me'.
One eBay seller said 'in the unlikely event of your fish arriving dead on arrival (DOA)' they offer a refund if the buyer sends a photo of the deceased fish in the unopened package - but only if the 'whole order is DOA'.
The private eBay seller that Ms Smith bought from has racked up dozens of negative reviews over the last 12 months from disgruntled customers.
Some claim their fish arrived in inappropriate packaging including cardboard boxes and plastic bags wrapped in paper and sent through Royal Mail, despite being prohibited.
Others complained about their fish arriving dead on arrival.
The seller admitted to supplying 'nano fish' - a trade term for small fish - in bottles but denied they were 'fizzy drink' bottles and said they didn't send fish via Royal Mail, despite the reviews claiming otherwise.
The private eBay seller that Ms Smith bought from has racked up dozens of negative reviews over the last 12 months from disgruntled customers
Posting the shocking pictures online, Tara Smith said: 'Has this group got a 'don't buy from' list because what I have just received has shocked me'
They said they adhered to eBay's policies including the 'safe, guaranteed overnight delivery', while referencing their thousands of positive ratings, while blaming complaints on 'competitors'.
Royal Mail's prohibited and restricted items list states that they are happy to post 'live creatures, insects and invertebrates including bees, caterpillars, earthworms, spiders and stick insects'.
However, it goes on to state 'Royal Mail do not accept fish or fish fry'.
eBay confirmed they allowed sellers to post live fish to customers providing they were under 5cm and could 'guarantee safe delivery'.
It's unclear if 'safe' refers to the parcel arriving or the fish surviving.
They did not state whether they would take any action against the seller despite the dozens of buyers claiming fish died en route to them.
An RSPCA spokesman said: 'This must have been really distressing for Tara and is a wholly inappropriate way to transport fish.
'We are a nation of animal lovers but sadly some animals, such as fish, can sometimes be seen as less important than others. But fish are sentient animals which means they have physical and emotional experiences so it's important to consider how our actions may impact them.
'We will be reaching out to eBay to discuss their current policies. No matter the size of an animal, they deserve our kindness and respect and transporting these fish in the post, or in such a small container, is completely unsuitable and would certainly have led to them being seriously distressed.
'We want to see a world where every animal is treated with kindness and compassion - and this, of course, includes our pet fish.'
Dominic Whitmee, chief executive of the Ornamental Aquatic Trade Association, said: 'We share this fishkeeper's dismay and annoyance at the treatment of her fish in this way.
Royal Mail's prohibited and restricted items list states that they are happy to post 'live creatures, insects and invertebrates including bees, caterpillars, earthworms, spiders and stick insects'
People praised Tara Smith for rescuing the fish she bought on eBay. People were shocked at the ill treatment
'Fish should never be sent through postal services, in fact it's against the law to do so, and they certainly shouldn't be transported in a plastic bottle like this.
'We would urge anyone buying fish online, particularly on these types of selling sites, to check how their fish will arrive and to refuse to buy from people who do not use an authorised courier.'
Royal Mail declined to comment further.
An eBay spokesperson said: 'We're sorry to hear about our customer's experience on eBay.
'We prohibit the sale of most live animals, however, aquarium or pond fish (up to 5cm long), snails, tadpoles or similar creatures are permitted as long as the seller can guarantee safe, overnight delivery. Aquarium or pond fish longer than 5cm may be listed with the 'Collection in person' option.'
MailOnline contacted eBay for further comment.