BOSTON/NEW YORK (Reuters) - Billionaire investor William Ackman's success at Mondelez International Inc (>> Mondelez International Inc) may depend largely on a calculation that nimbler rival Kraft Heinz Co (>> Kraft Heinz Co) will soon snap up the maker of Cadbury chocolate and Oreo cookies, sources familiar with Ackman's approach said.

Ackman's $20 billion hedge fund Pershing Square Capital Management disclosed late Wednesday it spent $5.5 billion (3.55 billion pounds) for a 7.5 percent stake in snack company Mondelez, making this the firm's biggest-ever bet.

It is the second time around for the activist investor and Mondelez. Pershing Square briefly owned the company in 2013 after its split from Kraft in 2012. Investors and analysts agree there are several routes to higher returns.

Costs could be cut further or the board may want to find a new chief executive to replace Irene Rosenfeld, 62.

But the most likely path could be a scenario where Kraft Heinz, created earlier this year, reunites with its former unit, the sources familiar with matter said.

The foods sector has already seen a raft of takeovers but a bid for Mondelez, with a market capitalisation of $74.6 billion, could be another megadeal in the space.

Analysts and investors familiar with Ackman's approach agree that the hedge fund mogul may have his sights on a merger strategy and Wall Street has been rife with speculation that Kraft Heinz would be a logical choice. But analysts also note there is work to be done integrating Kraft and Heinz still and that Kraft Heinz would be preoccupied for now.

Mondelez's shares - which initially jumped on news that Ackman's Pershing Square Capital would become the company's biggest shareholder - closed up just over 1 percent on Thursday at $46.81 amid doubts about whether a buyer could be found.

"We do not think a possible KHC/MDLZ merger is out of the question entirely, especially a couple of years from now," JPMorgan analysts wrote in a research note. "But near term we have questions about such a deal."

While Ackman prides himself on being a long-term investor who has been called patient, some people have said there are signs that he might be pushing for a quicker pace on making changes with this investment.

Kraft Heinz and Mondelez were once the same company and already know each other well, suggesting that due diligence for a possible acquisition may not take as long.

Plus interest rates are still low, meaning that a suitor could finance a takeover more cheaply now than later when the Federal Reserve prepares for a hike probably later this year. It would also mean that Mondelez' stock price might still be lower, making for a cheaper bid, than two years down the line when the price would presumably be higher.

Much of the speculation of who may put in a bid for Mondelez revolves around 3G Capital, the Brazilian private investment group that teamed up with Warren Buffett to create Kraft Heinz. Ackman has worked with 3G on a deal to take Burger King public and Buffett, the octogenarian investor, has suggested that he wants to do a few more deals during his career.

This might suggest there is an opportunity to join forces sooner rather than later, the sources said.

(Reporting by Svea Herbst-Bayliss and Lauren Hirsch; Editing by Cynthia Osterman and Lisa Shumaker)

By Svea Herbst-Bayliss and Lauren Hirsch

Stocks treated in this article : Mondelez International Inc, Kraft Heinz Co