Premier Miton (PMI), which had said in November it approached RMG's board to explore taking over the company will pursue plans for organic growth, while also exploring "tactical and strategic opportunities", according to CEO Mike O'Shea.

"We have concluded that there are insufficient commercial merits for our shareholders to make a formal proposal for the acquisition of RMG," O'Shea said in a statement on Thursday.

River and Mercantile also received a takeover offer last November from acquisition vehicle AssetCo, and said on Thursday those discussions were ongoing.

AssetCo already holds five million shares in R&M, representing about 5.85% of its voting rights, it said in November.

AssetCo chairman Martin Gilbert is a former co-CEO of FTSE 100 asset manager Standard Life Aberdeen, now called abrdn. Gilbert is also deputy chairman of River and Mercantile.

RMG said it was focusing on completing the 230 million pound ($316 million) sale of its Solutions pensions business to Schroders, and would update shareholders on its post-sale strategic plan in the spring.

Asset managers which actively manage funds have been bulking up in recent years to cut costs and fight off competition from rivals running cheaper passive, or index-tracking, funds.

RMG shares traded at 290.4 pence at 0840 GMT, their lowest since Premier and AssetCo announced their approaches. PMI shares fell 1.54%, while AssetCo was unchanged.

($1 = 0.7276 pounds)

(Reporting by Huw Jones and Carolyn Cohn; Editing by Rachel Armstrong, Elaine Hardcastle)