Farm leaders have called for a full investigation into an incident that saw more than 40 sheep slaughtered on the dockside at Ramsgate, Kent.
Livestock exports remain suspended from Ramsgate following the incident, which started when a lorry load of sheep failed an inspection on arrival at the port.
Two sheep were put down due to injuries – but a further two sheep drowned and another 41 were shot on veterinary advice.
The exact circumstances surrounding the incident remain unclear.
It is understood that the two animals drowned after they were unloaded from the lorry into a holding area.
Differing reports suggest the holding area either collapsed or the sheep fell into an underground tank through a manhole-type cover.
It remains unclear at what stage the 41 animals became lame – whether it was before, during or after they were unloaded from the lorry.
Livestock due for export are only allowed to travel following a veterinary inspection prior to loading.
There are also questions regarding the type of instrument used to slaughter the animals.
NFU chief livestock adviser Peter Garbutt said the union was calling for a full and frank investigation into the incident.
Pictures released to the media by the RSPCA after the incident showed a pile of dead sheep covered in more blood than would usually be expected.
Mr Garbutt said: “Concerns have been raised with us as to why contingency plans to deal with such events do not appear to have been followed by the bodies overseeing the trade.”
The NFU wants to know exactly who was present during the incident, the contingency plan which was in place and who made decisions regarding the animals.
Mr Garbutt said he would be concerned if the RSPCA had put pressure on government officials to sanction the slaughter of animals when more humane alternatives were available.
He added: “Exporting animals is a legal trade that is carried out throughout Europe and the health and welfare of the animals through transit is the top priority of farmers.”
Mr Garbutt said the NFU had met with the RSPCA to express its wish that the shortest route possible be used for animal transportation across the channel for that very reason.
“We have made it clear that the port of Dover would be the most suitable location, but as the boat being used is unable to dock there currently, Ramsgate is the next best option available.”
The RSPCA has said it was right that the animals were put down, describing them as “severely lame”.