The theft of sheep from farmers' fields has risen fivefold in the last 18 months compared with the last decade, the leading farmers' insurer said.
Thieves, who in recent years have reacted to global price rises for metal by stealing scrap and in oil by raiding farm tanks, have even been found to have butchered the meat in fields.
That led the insurer NFU Mutual, which covers two thirds of British farms, to warn potential buyers of the health risk of picking up a bargain which had not been hygienically slaughtered.
Typically, farmers were reporting that a couple of sheep at a time were being stolen - probably bundled into the back of a van to be slaughtered and sold on to friends.
But in some instances, gangs rounded up 100 animals and organised a lorry to whisk the sheep away to be sold on through illegitimate channels.
The problem was worst around Cumbria, North Yorkshire and County Durham.
NFU Mutual spokesman Tim Price said: "In the last 18 months we have seen the number of rustling thefts from farms increase fivefold.
"We have had about 10 years at a very low level and it has taken off again.
"We think there are two factors: the price of meat has shot up, and the recession. In the past when there is a downturn in the economy, rural crime goes up."
Thieves have even begun to steal hay after recent price rises due to last year's cold winter and a relatively poor crop in places this summer.
Mr Price said rustling was "probably one of the first crimes that took place".
"Some caveman probably snuck into his neighbour's cave and took away his goat," he said.
The insurer has asked people to look out for suspicious night-time activity on farms and urged consumers only to buy from reputable sources.