Packed tightly in the darkness, the squawking hens slither and slide amid their own faeces, some of them packed so tight they clamber on top of each another.

This shocking picture was taken covertly inside a long, narrow shed housing 16,000 hens.

‘In an instant, I was reaching for my handkerchief, so eye-stinging was the stench,’ says the photographer, an animal rights investigator.

Around 16,000 hens were pictured packed into a long, narrow shed on a farm in north Norfolk

The birds were 'covered in sores and mites' but the enclosure did not break any free-range regulations

‘The smell of ammonia was ghastly, but what I saw was even worse.

‘Many were bald. The ones I examined were covered in mites, some with sores. Several had wounds the others pecked. It was a revolting sight.’

Yet the scene he committed to video was not, as you might assume, on a factory farm.

Astonishingly, the hens he filmed are legally classified as ‘free range’. It is hardly the rural idyll that many consumers of free-range eggs might imagine.

Instead, the footage – filmed at night by a volunteer from Norfolk’s Hillside Animal Sanctuary in January this year – is a disturbing insight into what the poultry industry allows.

The hens are part of a 35,000-strong flock farmed by UKIP MEP and agriculture spokesman, Stuart Agnew in North Norfolk.

Nothing about the conditions under which the hens are kept is against current free-range regulations.

The 35,000-hen farm is run by UKIP MEP and agriculture spokesman Stuart Agnew, pictured

The footage, pictured, was filmed at night by a volunteer from Norfolk’s Hillside Animal Sanctuary

Indeed, an RSPCA investigation found everything to be in order. And perhaps that is the most shocking thing of all.

The hens are quite legally branded free-range because they have access to an outside run through ‘pop’ holes punched in the walls of their shed – provided they can push their way through the thronging mass of flapping wings and pecking beaks.

So they are deemed to have ‘outdoor space’, irrespective of how little time they actually spend in it.

Eggs from the crowded hens are supplied to Nobel Foods, which owns two brands of free-range eggs – Eggs For Soldiers and the Happy Eggs Company, both sold in leading supermarkets.

The price of Eggs For Soldiers includes a donation to military charity Help for Heroes.

A spokeswoman for Nobel Foods categorically denied that Mr Agnew’s eggs are used in its Happy Eggs Company brand, but no one from the company could say which brand his eggs were sold under.

It is understood that Mr Agnew is not the only supplier to Nobel Foods. Each year, British families eat 11.5 billion eggs, of which 10 billion are produced in the UK.

The free range market is huge, accounting for half of all eggs in supermarkets. Britain has the largest free-range flock – 15 million birds – in Europe.

But as demand has grown, so has the size of free-range farms. Under the industry’s quality assurance regulations, free-range farmers can house 16,000 hens in one building.

Individual flocks, separated by wire fencing, can be up to 4,000 strong. Under EU rules, nine hens can be crammed into every square metre of floor space.

Outside, there must be at least 10,000 sqm of space for every 2,500 hens. But there are no rules to say how often the birds must visit the outside world.

That is up to the farmer. Mr Agnew’s flock are labelled ‘RSPCA Assured/Freedom Food’ which means they are subject to stringent welfare standards and regular checks.

When the RSPCA was alerted to the footage, it launched an immediate investigation and concluded it was satisfied with the conditions.

The eggs are also used in the Eggs for Soldiers brand, which includes donations to Help for Heroes in the price

Both the RSPCA and Nobel Foods pointed out that the poor condition of some birds captured in the footage was due to an outbreak of a disease called enteritis, which is common in poultry.

The RSPCA’s head of farm animals, Dr Marc Cooper, said: ‘We checked to see that they had been seen by a vet and were satisfied they were.

'We deemed the birds looked bright. Yes, some had mites but it was being treated.

‘We did feel the ammonia level was high in one area but that was addressed through ventilation.

'The filming was done at night when the ‘pop holes’ through which the birds gain access to the outside were closed.

'We investigated thoroughly and were satisfied that the producer had done everything he could and should. If he hadn’t, we would have taken it further.’

Nobel Foods, one of the UK’s leading supermarket suppliers, say it, too, was initially concerned.

‘We always take the welfare of free-range hens very seriously and we are saddened to see the condition of these hens,’ says its press spokeswoman, Amaya Alvarez.

The RSPCA inspected the farm but found there had been no breach of any animal welfare regulations

‘An independent RSPCA and Freedom Food inspection of the flock took place immediately after we were alerted and the farmer has not breached any guidelines on animal welfare.

‘In August 2015, he immediately alerted his local vet when he noticed some of his birds were unwell.

'The treatment and condition of these birds pose no risk whatsoever to the end consumer of eggs. Eggs from the site will continue to be graded and packed in the normal manner.’

Not everyone, however, is convinced. Respected vet Amir Kashiv believes the conditions in this and some other hen houses are inhumane.

‘While the birds here have considerably more room to move than battery hens, the grim and squalid conditions are a far cry from the image and impression conjured up by the terms “free range” and “Freedom Food”,’ he insists.

‘I have watched footage filmed at night at Mr Agnew’s farm between December 2015 and January this year,’ he said.

‘The hens are housed in considerable density and seem to be mostly on barren wooden slats flooring; this surface is unnatural and damaging to the hens’ feet and they could also get a leg stuck between the slats.

‘One hen is seen with half her body stuck between the edge of this flooring and a wall.

'A few hens are also seen in the space underneath the slats – presumably they fell down there through some gap – where they would be covered by the droppings of the “upstairs” hens.

‘A few hens are seen in obvious distress, unable to move and in breathing difficulties; the latter would be exacerbated by the very high ammonia concentration measured during the filming.’

Some free-range farmers do, of course, keep their poultry in a more traditional, less densely packed environment.

Professor Christine Nicol, a leading animal welfare expert at Bristol University, has produced a report revealing that free-range is no guarantee of animal welfare – and that birds may suffer less if they are kept in cages.

‘The problem is that the management of free-range systems in the UK at the moment is so variable, that although you get some brilliant farms, you also get some that are really not good.’

Mr Agnew said: ‘My farm has always adhered to the strict standards required by the RSPCA.