In the past month, 22 men and a young boy have been arrested by police in raids at two illegal fights. Previously, there had not been an arrest involving cockfighting for a decade.
The head of the RSPCA's Special Operations Unit believes a secret network is operating across the country, trading in birds and fighting equipment. The RSPCA has evidence to show that organisers and followers of cockfighting travel hundreds of miles for meetings.
The "sport" is difficult to infiltrate. Unlike dog fighting - which tends to attract known criminals and, therefore, informers - cockfighting enthusiasts are usually "country sportsmen", such as farmers. Information about fights and the sale of birds is only passed on to people who are trusted. Hundreds, even thousands of pounds are placed in bets.
In the latest incident, on Monday, police and RSPCA officers raided a travellers' caravan site near Erith, south-east London. Sixteen men and a boy aged eight were arrested. Seven dead birds and metal cockfighting spurs were seized.
When the authorities arrived, they found two birds fighting in a pit and a number of live birds waiting near by. Officers found a fighting manual at the site which gave details of the rules of the contest.
An inspector said: "The sight of birds being ripped to shreds is one that I know will stay with the officers for a long time. Many people think cockfighting died out in the Middle Ages, but it still takes place somewhere in this country every week." The 16 arrested men have been charged and bailed.
Three weeks earlier, six people were arrested in a shed on allotments in Kelloe, Co Durham. They found 14 dead cockerels, about 40 live birds, spurs, a weighing machine and a board listing birds' names and betting odds. About a dozen men escaped capture.
It is illegal to organise or attend a cockfight. Cockfighting was outlawed in 1849 and offenders face a maximum penalty of £5,000 and six months in prison under the Protection of Animals Act 1911.
The men who train the birds - known as "cockers" - use diet and exercise to develop their fighters. Instruction manuals, such as Grit and Steel, are available through mail order catalogues in the United States. Steel spurs, usually about 2in long with sharp pointed tips, are tied to the bird's feet to inflict maximum damage to the opposing bird. They are also available by mail order from America, Mexico and the Philippines. Birds' eggs and fighting cockerels are also brought in from abroad.
Chief Superintendent Don Balfour, head of the Special Operations Unit, said: "The trainers approach the conditioning of a fighting bird rather like a trainer of an athlete.
"The fights are highly organised with special rules, a referee and timekeepers. Because it is such a secretive world, it is hard to know how widespread it has become, but I'm convinced it goes on all the time."
The birds can also fight "naked heel", in which the natural bone is used. The effect of the steel is to stab the opposing bird to death. The cockerels usually try to leap in the air and stamp down on to their opponent. They also attempt to peck their rival's head and eyes.