One evening last week, under cover of the fading light, I climbed over a wall to get into the grounds and corrugated iron sheds of a small farm in the English countryside.

With a cameraman, I waited until dusk. We hoped the farm workers had clocked off by then, but we couldn’t be sure. If we were caught, we faced being prosecuted for trespassing.

It was a nerve-racking mission, but one worth taking to shine a light on the sickening conditions in which pigs are being kept in Britain.

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Tracy Worcester, an animal welfare campaign, armed with her camera during her recent undercover mission

What I found was not just distressing, it was horrific. Through the darkness I could see the pigs crammed into small pens on bare slats (for draining the effluent) or on concrete. There was no soft material such as straw to lie on, which they should have by law.

Sows with suckling piglets were in crates so small they couldn’t turn around. They will stay like this for three or four weeks until their piglets are weaned. This is tantamount to torture.

Pigs that had died of disease or neglect were left lying on the ground – it is probably too expensive to clear them away – and, a stomach-churning sight, there was an open bin full of dead piglets.

All of the pigs, even the tiniest newborns, had their tails docked. This is a bad sign and, again, against the law. It is a practice only used when pigs are so stressed they start biting each other’s tails.

We pride ourselves on being a nation of animal-lovers and tell ourselves our rules on pig welfare are among the strictest in the world. But we need to face the truth about the pork we buy in shops and supermarkets.

The horror I filmed is by no means unusual. Far from it. It is typical of farms throughout the country, so much so that I am not identifying this particular farmer.

A dead pig is left to rot on the floor next to frightened animals in a tiny pen

He is not the enemy, but the victim of a system that treats animals – highly intelligent in the case of pigs – as mere commodities.

Much of the pressure to cut costs is driven from abroad. The fact is that the Great British Bacon Butty is often not British at all. Some 54 per cent of our pork is imported – mostly from pig factories in Germany, Denmark and the Netherlands – and made in conditions that would be illegal here.

Some meat might come from as far afield as Chile, where I filmed the local people’s successful battle to close the world’s biggest pig farm, housing an astonishing 2.5 million animals. These cheap imports from giant corporate animal factories undermine British farmers who, with higher welfare standards and smaller farms, cannot compete.

Pig factories taking advantage of cheap labour and lax welfare laws are pushing British farmers to the wall.

Cutting corners in animal welfare becomes the only option to avoid bankruptcy. In the past 15 years we have lost half of our sow population, with many of our small and medium-size farms being forced to close. Once gone, they are gone for ever.

Yet we can act. By using our power as consumers, we can choose pork that carries the RSPCA Assured label, is free range, outdoor bred or organic – and change the system. The power is in our purse.

Inquisitive pigs in dark, barren pens greet Tracy Worcester on her undercover mission

My eyes were opened to the plight of pigs in 2005 when I was invited to film undercover at a factory in Poland owned by Smithfield Foods, the largest meat-producing and processing company in the world.

In one shed, thousands of pigs were crammed in tight pens. Squealing in distress, some had gnawed on the steel bars until their gums were bleeding. The smell of faeces was indescribable. Following the flies to a large plastic bin in the yard, we found piles of dead pigs and piglets that hadn’t survived. Some were floating in an effluent storage lagoon.

What I saw was so appalling I vowed to put all my time and resources into informing the British public about this barbaric system and preventing similar factory farms springing up here. My organisation, Farms Not Factories, was born.

Having acted in the hit 1980s TV series C.A.T.S Eyes and being married to the Marquess of Worcester, there was a lot of media interest in me. For the past ten years I have used my title and connections to raise money to make films to show the horrors behind the closed doors of pig factories.

This marchioness does a lot of clambering over walls and razor- wire fences and creeping around stinking pig factories.

I have kept pigs and they are charming creatures. Social, affectionate, inquisitive and, contrary to the stereotype, very clean. In the wild they form stable family units led by a matriarch, much like the social structures of elephants. As with most mammals there is a strong bond between a mother and her young.

The pigs are left with desperately little room to move and with no straw

Studies have shown pigs to be quick learners, able to respond to commands much like dogs and perform complex tasks like chimpanzees.

Animal factories are not only bad for these sensitive, fascinating animals – they also pose a serious risk to human health. Keeping animals in unnatural and unhygienic conditions promotes disease, so factory pigs are routinely given doses of antibiotics.

Alarmed at increasing human resistance to antibiotics, doctors and hospitals are cutting back. But at the same time their use by factory farms is increasing. In the UK, 45 per cent of antibiotics sold are to treat animals. In the US more than 80 per cent of all antibiotics are used by agribusiness.

This is fuelling the proliferation of antibiotic-resistant bacteria and making some human diseases more difficult to treat.

Two sausages from a factory-farmed pig costs the same as one and half sausages from a farm where they are raised humanely. Surely avoiding animal cruelty and saving antibiotics is worth half a sausage?

I grew up in the countryside near farmers who loved their animals. I know that if it were economically viable, farms like the one I visited last week would prefer to treat their animals well.

The Government could impose high tariffs on cheap imports to guarantee the British farmer a fair price, but it is reluctant to do so. That leaves it up to us to act. You and me. The consumers. By buying higher-welfare meat, we are ensuring the survival of our farmers.

The most recent campaign by Farms Not Factories – endorsed by celebrities such as Dominic West and Helen McCrory – touched people around the country. We asked them to take a selfie with their nose turned up, hashtag it #TurnYourNoseUp and post it online where they can watch a film about animal factories.

Each time we buy pork we vote for the system that produced it. Vote for pigs raised in this country on farms where they are allowed to roam and feel the sun on their backs, and where our farmers receive a fair price for good animal husbandry.

It will make you feel even better when you bite into a Great British Bacon Butty.