At least one quarter of meat on sale in the UK comes from farms that do not have to meet national standards for animal welfare, a Guardian investigation has found.

The poor conditions in which the animals can be kept include greater crowding in chicken and turkey sheds, cage-like pens for female pigs and physical castration with no anaesthetic for boars.

The UK has relatively high standards of animal welfare compared with the rest of the world, including the European Union. However, there are no restrictions on importing meat from countries that do not impose such standards, where costs are often lower. There are renewed calls from consumer and farming groups for better labelling of products and action to bring standards into line, at least across the EU.

"Sometimes people may be making false assumptions about products, sometimes they may have genuinely lower standards – I don't think people know what the situation is," said Sue Davies, chief policy adviser for the consumer lobby group Which?.

Kevin Pearce, head of food and farming at the National Farmers Union (NFU), said some overseas farmers would be required by retailers to meet higher standards than their national minimums. However, there is particular concern over the quality of imported food used in restaurants, pubs, canteens and other catering outlets, which now accounts for half of the money spent on food in the UK, said Pearce.

"If consumers know anything about it, they probably think all the standards are the same," he said. "It's not about farmers whingeing, we want to be able to compete fairly. If the customers say 'that's the standard we want', we want to do our best to produce it. Where we have a problem is if the price is too high or the supply too short they'll go elsewhere to get it."

Public concern about the costs of modern food has been growing after a number of health scares, high-profile books and, last week, the UK premiere of the Oscar-nominated documentary Food Inc, which claims to expose the "highly mechanised underbelly" of the industry.

The NFU has previously complained that the differences in standards are unfair to UK farmers, for example claiming that half of the pork market went overseas when sow pens and tethering were banned.

However the scale of the problem has been revealed by a Guardian analysis of the most recent full-trade figures available, for 2007. It found that:

• More than half of bacon sold in the UK comes from the Netherlands, Denmark, Germany and Italy, where farmers can keep sows in smaller pens and for longer periods.

• 43% of other pork products come from Denmark, Germany the Netherlands, Belgium and Luxembourg, where the same poorer conditions on pig farms are allowed.

• One quarter of poultry sold in the UK comes from seven European countries and Brazil, which allow higher stocking of chickens and do not force farmers to use more comfortable dry bedding.

• 3% of beef is imported from Brazil, where practices including hot branding, castration and dehorning of cattle can be carried out without anaesthetic. Products from those major importers accounted for about one quarter of all meat sold, by weight, in the UK that year. Total imports of pork, poultry, beef and veal made up one third of all meat sales, and it is likely that some of the remaining imports came from smaller trading countries also with lower standards.

Concerns have also been raised about imports of lamb and mutton from Australia, where mulesing – cutting away the woolly skin around the buttocks to prevent pests breeding – is common.

The Soil Association, the trade body for organic farmers, has raised concerns that imported organic pig and poultry meat often comes from countries with lower organic standards than those it sets in the UK. The NFU said it wanted both better labelling and education about the standards for imported meat on sale, and fairer trade rules to create a level playing field between UK and overseas farmers. Davies said Which? was already part of a lobby pressing the EU to make retailers label food more clearly with the country of origin. Most food does not have to be labelled, and often labels only show the country where the food was last processed.

"There are three issues: being clear about the relative quality of our standards and whether it's beneficial to be buying British, or not; whether we should have a simpler system across the EU, even should they have the same standards; and there's a lot of consumer demand to have information about the country of origin," said Davies.

UK meat key concerns

Poultry: Most UK farmers are limited to a maximum of 38kg per square metre of space – 15 birds at their fattest – compared with one third more than that in the EU. UK standards for access to drink and feed, and healthy bedding are also higher, said the NFU.

Pigs: Farrowing crates are used in the UK for a few weeks after piglets are born, something which hugely reduces young mortality; most countries use significantly smaller sow pens and the animals are kept in them most of the year.

Beef: The chief concern is imported meat from Brazil, where cattle can be castrated up to six months old, dehorned and hot-branded, all without anaesthetic – all of which are banned in the UK.

Organic: The biggest UK organic body specifies that pigs are fully free range and poultry flocks no bigger than 500 birds to increase access outdoors and reduce bullying. Many other certifiers allow pigs to be often kept indoors and unlimited chicken flocks.