Budgens will be the first to benefit, with the new lines appearing in-store from 14 September, but MBL says its other fascias will soon follow.
The move follows a major review of its fresh fish range, which found the focus on branded product was not getting the right message across to shoppers and thus not stimulating adequate sales growth.
Buyer Phil Punter said: "Fresh fish is a high growth category, with health issues and consumer awareness of the benefits of Omega-3 currently driving demand for oily fish. For several years we've only had a branded offer, but by moving to own label we have been able to create consistency across the category, add breadth of range and give fresh fish the focus it deserves in-store."
The new range will mirror the three-tier 'good, better, best' offer available in own-label fresh meat, which was relaunched in the spring, but MBL is not abandoning branded fish altogether.
It plans to keep on some premium brands to appeal to shoppers with higher budgets. The basic range will include everyday products such as Budgens breaded white fish and Scottish salmon steaks.
The 'better' offer will include fillets, oily and smoked fish and fish cakes, while 'best' will comprise own-label tuna steaks supported by the Bleikers range of smoked fish.
The own-label format allows MBL to communicate messages on provenance and sourcing, so, for instance, all cod and haddock is line-caught and the Marine Stewardship Council mark will appear on packs of fish cakes.
MBL will also be supporting National Seafood Week (from 6 to 13 October) with additional promotions and support in the Budgens consumer offers leaflet.