The latest enquiry by Which? watchdog reveals that many famous brands of ham contain as much water and additives as meat, but consumers are left in the dark as to their ingredients.
The watchdog, funded by the Consumers Association, urges the labeling loophole be closed to protect customers paying over the odds for low-grade meat products.
By law producers have to declare the percentage of pork in packaged ham, and they must indicate water as an ingredient if it constitutes more than five per cent - but they don't have to say how much water has been added.
This has provided a window of opportunity for retailers, with Marks and Spencer's Danish Wafer Thin Ham, Sainsburys Basic Cooked Ham, Asda's Rollback Smart Price Thin Sliced Ham and Tesco's Value Cooked Ham all containing between 16 and 25 per cent water.
"We want food companies to spell out exactly how much water they've pumped into their ham on the front of packs, so people can easily see how much they're paying for meat, and how much for water and additives," said Which? magazine editor Malcolm Coles.
"As long as a legal loophole lets companies avoid admitting how much water ham contains, consumers risk being sold a pig in a poke."
Tests revealed that canned and re-formed hams were the worse offenders, containing between 21 and 37 per cent water.
In 1985, the British Meat Processors Association (BMPA)and the Local Authorities Coordinators of Regulatory Services (LACORS) drew up a voluntary industry code to assist manufacturers in the labelling of ham and ham products.
The code of practice on the labelling of re-formed and cured meat products lists three categories of ham for labelling purposes: ham, formed ham and re-formed ham.
But there are no plans to push for more stringent legislative rules.
Peter Scott, director of BMPA, indicated that the organisation does not see a problem with ham labelling. He said: "Apart from the fact that all ham is safe and wholesome, the key factor here is price...This underlines the fact that consumers will get what they pay for, as declared on the label. The print and size of labels and their positioning will remain a matter of conflicting demands."
Ham is meat that comes from one whole, cured leg of pork. 'Formed' ham is made of several pieces of meat from more than one leg of pork, shaped into a joint before cooking and slicing.
'Re-formed' ham is meat that has been "rearranged" and comprises muscles of more than one leg with the addition of finely comminuted meat or meat emulsion, before being shaped, cooked and sliced.