Lidl has become the UK's third biggest retailer of sustainable seafood behind Sainsbury's and Waitrose - more than doubling its range over the last year, the Marine Stewardship Council (MSC) said.
More than half of Lidl's wild-caught seafood range - 79 out of 148 products - now carries the MSC's blue tick ecolabel, which tells shoppers that the item has been fished sustainably and is traceable from ocean to plate.
Sainsbury's currently stocks 200 MSC-labelled products - more than twice the number of any other UK retailer - while Waitrose carries 86 sustainably fished items.
But Lidl's rival Aldi has also increased its number of blue tick products to 54, "a far bigger range than most of the 'big four' and more than some grocers known for their ethical sourcing" such as Marks & Spencer and The Co-operative, the MSC said.
The latest figures show three of the UK's biggest supermarkets - Asda, Morrisons and Tesco - along with M&S, now sell fewer MSC certified products than they did a year ago.
Toby Middleton, MSC's programme director for the UK and North East Atlantic, said: "Sainsbury's and Waitrose have long been committed to sourcing seafood that has been independently certified as sustainable, and it's great to see Lidl and Aldi following suit as they grow their market share and seek to get more customers through their doors.
"As well as helping to safeguard our oceans for the future, it means shoppers can find a wide range of sustainably sourced seafood at a price to suit all budgets - whether they're picking up a pack of fish fingers for the family's evening meal, or shelling out for a lobster for a special occasion."