A "potentially disastrous" backlog of thousands of pigs is building up on East Anglian farms due to labour shortages at meat processing factories, said industry leaders.

A shortage of workers is affecting many business sectors following the post-Brexit changes to immigration rules, which have ended free movement across borders and forced many Eastern Europeans - which food and farming firms rely on - to seek employment in other countries.

Last week, the National Pig Association warned consumers it could "lead to empty retail shelves, which is likely to result in more imports".

But it is also affecting animals on farms.

With major meat processors reporting a 15-25pc drop in their capacity, pigs are backing up on farms - gaining more weight than their contracted specifications, and adding to the cost and manpower required to keep them healthy and avoid welfare problems.

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A similar situation arose earlier this year as a result of Covid-related staff shortages.

In response to renewed crisis warnings from the pig sector, the government said it is working with the industry and the Department for Work and Pensions to recruit UK workers into food and farming jobs.

One farmer, the manager of an integrated pig breeding and finishing firm in South Norfolk, who did not wish to be named, said his business was dealing with a surplus of 6,000 pigs in the last four weeks.

"We are doing 6,000 pigs per week, but with the five processors we supply there has been a 25pc reduction in their processing capacity across the board," he said. "In four weeks we have lost a week's processing, so these pigs are backing up - we have created 6,000 pigs extra on farms.

"We have got a very tired bunch of farmers who are contract-finishing for us. They have not had any time off since October and we have to keep asking favours of them just to get the pigs housed, in less-than-ideal situations.

"We do our utmost to cater for 100pc of the animals' needs, but unfortunately there are issues with over-crowding. In these situations, it is not about the finances, it is about the staff and animal welfare. But the company is struggling.

"The government seems to keep suggesting using British labour, but people need to acknowledge that is not going to happen."

Nick Allen, chief executive at the British Meat Processors Association, said: "It is a serious concern. The major processors are all struggling to run at full capacity. There is a range of figures, but they are reducing their kill rate by between 15-25pc.

"The problem at the beginning of the year was that two major plants went down with Covid for two weeks, but we knew we could come back from that. This is more worrying because everyone is looking at this, not knowing where this labour will come from to to increase their capacity again.

"The farmers are right to be concerned, and we are concerned. There just does not seem to be the labour out there."

Norfolk pig farmer Rob Mutimer, of Swannington Farm to Fork, who is also chairman of the National Pig Association (NPA), added: "The industry has had a really tough 18 months - people's morale is at an all-time low, and now we are facing another potentially disastrous situation with not being able to get pigs away from the farm to the abattoir, which is leading to the same problems we had during the winter.

"It is financially burdensome, and it is very hard for staff on the farms having such a big workload and trying to prevent welfare issues on pig farms.

"This is a political problem. People are trying hard to recruit staff and they are paying them more money, but unless the government accepts that we need more workers in the system, this problem will remain."

The government said although its points-based immigration system has a route for skilled workers to enter the UK, it "will not introduce a general low-skilled or temporary work route". Instead it aims to encourage all sectors to "make employment more attractive to UK domestic workers through offering training, careers options, wage increases and to invest in increased automation technology".

A Defra spokesperson said: “We understand the challenges that the industry has faced in recent months, and Defra has been collaborating with the pig and processing sectors during this time.

“We are working with the Department for Work and Pensions (DWP) to raise awareness of career opportunities within the food and farming sectors among UK workers.

“We are also working closely with the Home Office to ensure there is a long term strategy for the food and farming workforce beyond 2021."