The MCS is advising consumers to avoid vulnerable and overfished species such as shark and deepwater fish.
It is also urging retailers to source traditional favourites such as cod and haddock from sustainable fisheries listed on the FISHonline.org web site.
The most environmentally sound seafoods include mussels, oysters, Alaskan salmon and diver-caught scallops, said the MCS.
The MCS gives fish on the list a rating from 1 to 5 that allows the user to easily identify the status and sustainability of each species at a glance.
A rating of 1 is awarded to the most sustainably harvested seafood, including fish from certified fisheries. A rating of 5 is given to those fish that the MCS recommends be avoided. Ratings 2, 3 and 4 indicate increasing levels of concern.
FISHonline.org is supported by Marks and Spencer.