Discounters are set to face greater scrutiny over how they source tinned tuna as Greenpeace expands its supermarket tuna survey.

The NGO said its survey would now include Aldi, Lidl, Iceland, Booths, Budgens and Ocado. It has to date covered only Tesco, Sainsbury’s, Asda, Morrisons, Waitrose, The Co-op and M&S.

Greenpeace supporters will visit the retailers under scrutiny, where they will record which types of tinned tuna are sold and what fishing methods are used.

Greenpeace’s campaign had led to more sustainable tuna becoming available in own label, but new, less sustainable branded lines were now “creeping back,” said oceans campaigner Ariana Densham. It was time all retailers - including discounters - committed to stocking only sustainable branded and own-label tuna.

“Sainsbury’s effectively already applies this by only stocking brands with sustainability commitments,” she said. “Other supermarkets also have sustainability commitments but have not yet made an explicit commitment they will not introduce unsustainable brands in the future.”