NEW YORK (Reuters) -Post Holdings is working with investment bankers to explore a possible transaction with Lamb Weston Holdings, as the frozen potato products maker faces pressure from an activist investor to sell itself, two sources familiar with the matter said.

For packaged goods company Post, which makes Grape Nuts cereal and Bob Evans mashed potatoes, Lamb Weston has long been an attractive business that it considered buying in 2015 when the french-fry maker was still part of Conagra.

Now Post is again exploring ways to own Lamb Weston and is consulting with bankers in the tight-knit food industry, said the sources who insisted on anonymity because the discussions are private.

Post has a market value of roughly $7 billion and the sources said there would be considerable synergies with Lamb Weston, which has a market value of roughly $10.5 billion, if a combination were to occur.

A representative for Post declined to comment while a representative for Lamb Weston was not immediately available for comment.

Two months ago activist investor Jana Partners said in a securities filing that it built a roughly 5% stake in Lamb Weston and wants the company to improve operations and capital allocation and consider selling itself. The news sent the stock price up roughly 10%. Since January, Lamb Weston's stock price has dropped 31%.

Jana is working on the investment with holding company Continental Grain and has identified several industry executives, including Lamb Weston's former executive chairman Timothy McLevish, as potential board candidates.

Wall Street analysts have said Cargill, Post, Tyson Foods and Kraft Heinz could be hypothetical buyers.

Jana has a history with Lamb Weston and in 2015 pushed Conagra, which owned the business at the time, to divest it. Lamb Weston became a publicly listed company in 2016.

Now the activist is pushing Lamb Weston to make changes or possibly face a board challenge.

Jana has extensive experience in the food sector, pushing for improvements most recently at pet food manufacturer Freshpet, and in earlier campaigns at companies including Whole Foods, Pinnacle Foods, Safeway and Conagra.

(Reporting by Svea Herbst-Bayliss and Abigail Summerville; Editing by Chizu Nomiyama)

By Abigail Summerville and Svea Herbst-Bayliss