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The RSPCA says it has not been given permission to monitor live animal exports since the controversial trade resumed at the Port of Dover.
The charity wants inspectors at the Kent port to safeguard the welfare of animals as they are shipped for rearing and slaughter across the English Channel.
But it claims the Dover Harbour Board (DHB) has not granted approval for it to monitor animals on Barco de Vapor's livestock export ship MV Joline.
The RSPCA has now called for pressure to be brought to bear on the DHB to allow inspectors in amid claims that animal exporters have historically flouted UK and EU rules.
The MV Joline, which has capacity for seven lorries, started weekly shipments of live animals from Dover to Calais on May 2 in the face of protests from animal welfare campaigners.
The DHB has defended allowing the shipments to pass through Dover, having previously gone from Ramsgate, by saying it is "duty bound'' to facilitate the legal trade and has faced legal action after refusing in the past.
Any change to the legality of live animal exports can only be addressed by politicians, officials said. But they hoped that the comparatively shorter sea crossing from Dover would bring some relative benefit to the animals' welfare.
RSPCA chief executive Gavin Grant said: "We were extremely saddened to see that live exports have resumed from the Port of Dover, a move that could see an increase in the number of animals that will unnecessarily have to endure long journeys to the Continent.
"Like Ramsgate, Dover does not have facilities at or near the port to deal with emergencies and our inspectors have not been granted permission to the port to monitor the welfare of the animals.''
He added: "Unfortunately, the animal transport industry has frequently shown violations of UK and EU regulations and RSPCA presence would bolster public confidence and ensure animal welfare issues are being fully addressed within the port environment.''
The DHB said it had written to Mr Grant last month pointing out that the board had been taken to the High Court when it previously refused to allow the trade to pass through the port.
In the letter, the DHB said: "The High Court has made it crystal clear that ports, and DHB in particular, must facilitate this legal trade. In short, shippers may choose ports but ports may not choose shippers.''
It added: "As it is currently a legal trade, the only way it will stop is if there is a change in the law. This means that it is a situation that ultimately only politicians can change.''
Officials said they will be "rigorous'' in demanding that all aspects of animal health and welfare while in the port are complied with and that the vessel meets all requirements.
The DHB also asked Mr Grant to approach the Animal Health and Veterinary Laboratories Agency (AHVLA) about carrying out inspections at Dover.
The letter concluded: "We understand that we may not have addressed all your concerns but we hope that you can appreciate the inflexible legal position that DHB, and indeed any other port approached by a livestock carrier, is in.''