Shoppers are being overcharged for cheaper fish being sold off as cod, according to new research.

Some 7% of cod was mislabelled in the UK and 28% in Ireland, a study published in the scientific journal Fish and Fisheries found.

Less expensive species, including pollack and whiting, are labelled and sold as cod and a threatened species of cod was also being sold as "sustainably sourced" in one supermarket, the study said.

Some 88.6% of all mislabelled cod products identified from both Ireland and the UK were smoked, breaded or battered, because this can conceal the appearance, smell and the taste of a fish fillet, researchers said.

Scientists at University College Dublin used DNA barcoding techniques to genetically identify 226 cod products purchased from supermarkets, fishmongers and takeaway outlets across Ireland and the UK.

They compared the results against the product labels and found in 37 of the 131 cod products purchased in Ireland, and seven of the 95 purchased in the UK, the products were shown not to be cod.

"We found mislabelled cod products in each type of outlet, and identified that most of the mislabelled cod products were actually less expensive fish species substituted for cod and sold to consumers at a price premium," said Dr Stefano Mariani, from the university's School of Biology and Environmental Science.

The research also uncovered a more subtle form of mislabelling with threatened species of Atlantic cod mislabelled and sold as "sustainably sourced" Pacific cod.

All of the cod products mislabelled as "sustainably sourced" Pacific cod were purchased from a single supermarket chain that operates in both Ireland and the UK. The research did not name the supermarket chain.

Cod is the most popular whitefish consumed in Ireland and the UK, and the demand remains high despite the fact that local Atlantic cod stocks have largely been depleted and much of the cod is now imported.