Robyn Kippenberger, National Chief Executive of the Royal New Zealand SPCA said: "Hams marked with the SPCA Blue Tick are the only ones with an animal welfare guarantee.

"Many New Zealand pork producers are still choosing to confine their sows in inhumane sow stalls and farrowing crates and rear their pigs in barren concrete enclosures. Neither "100 per cent New Zealand" nor the New Zealand Pig Industry "pig care" certification guarantee the pig that your ham came from had welfare any better than most imported pork."

The SPCA Blue Tick animal welfare label is awarded only to farmers who do not use sow stalls or farrowing crates in their pork production and cage free production of eggs. The SPCA standards are upheld by regular audits to ensure farms adhere to these requirements.

Ms Kippenberger said: "The SPCA Blue Tick is a consumer guarantee of good animal welfare standards and these are the only independently audited animal welfare hams on the market.

"Despite the New Zealand origin, your ham may have come from a pig that was kept in barren concrete pens, born from a mother who spent most of her life in a cramped sow stall or a farrowing crate where she could only stand up or lie down, unable to turn around and with no contact with her piglets. SPCA Blue Tick farmers do not use these inhumane practices and we urge shoppers to support them and choose SPCA certified Blue Tick ham if you want to ensure your pig, and its mother, was a merry one."

In the 2010 review of the animal welfare pig code the New Zealand pig industry fought hard to extend their use of sow stalls and farrowing crates. Both systems severely restrict the sows movement for the majority of her life. The pig code now states that sow stalls will be phased out by December 2015 but farrowing crates are set to stay until an alternative intensive method is found.