Animal welfare officials are investigating "deeply distressing" footage of pigs being shot following a disease outbreak at a Norfolk farm.

The farm, between Feltwell and Brandon, became the centre of a foot and mouth scare on June 23 - until government vets ruled out the devastating livestock infection the following day.

After Defra completed its investigation into possible notifiable diseases, the farm took the decision to cull the infected animals by shooting them.

Although firearms are an accepted method to do this, animal rights campaigners claim the way it was carried out was a "blatant breach of animal welfare rules set to ensure animals die humanely".

Drone footage has been filmed showing pigs corralled into a pen of straw bales and shot at - some are seen writhing and convulsing on the ground afterwards.

RSPCA Assured said it is "difficult for us to tell from the aerial footage whether all the correct regulations and RSPCA welfare standards were followed".

As a result, the RSPCA has temporarily suspended the farm from its assurance scheme "as a precaution" while it investigates.

The farm is part of the Wayland Farms group. A spokesman for the company said: "Unfortunately a cull did take place at the farm, but it was conducted with the advice of official vets and under the full supervision of vets at the time of the cull."

The undercover filming was part of a series of farm investigations funded by green energy supplier Ecotricity, whose founder Dale Vince said: "Again and again, we see evidence of the most awful treatment of animals on farms.

"Some of it is allowed by the rules (incredibly), some of it is not."

Lex Rigby, head of investigations for vegan campaigns charity Viva!, added: “Red Tractor recommends that killing procedures should be carried out away from other pigs to reduce any unnecessary stress.

"Yet, since there are between 10-15 in the makeshift pens during culling at this farm, I’d suggest that causes these animals serious torment and violates their freedom from fear and distress - something which is supposedly protected by the Animal Welfare Act 2006."

Rebecca Lenik, senior public relations manager for the RSPCA Assured scheme, said they believed the cull was carried out on the advice of Defra and in the presence of a vet and a representative from the Animal and Plant Health Agency.

"This footage is deeply distressing to watch," she said. "However, sadly there are sometimes occasions when it’s necessary to humanely cull animals in certain circumstances to prevent the spread of disease, or potential disease, which we understand was the case in this situation.

"However, it is difficult for us to tell from the aerial footage whether all the correct regulations and RSPCA welfare standards were followed.

"Therefore we have suspended the farm as a precaution whilst we investigate. This means the farm cannot sell or market any products as RSPCA Assured.

"Our investigations are ongoing and we are unable to comment any further at this time."

Defra said culling decisions taken by the farm would be based on the clinical picture on the farm, test results, advice from disease experts and their private vet.

In this instance, it said it would be the responsibility of the local authority to investigate welfare claims.

A Norfolk County Council Trading Standards spokesperson said: “While we hadn't had any reports about this specific incident, generally speaking if we are made aware of a concern such as this we would look into it, in liaison with enforcement partners, to see if any offences have been committed.

"This is what we're currently doing in this case.”