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New hint in Warren Buffett succession sweepstakes

Tuesday marked a step up for Ajit Jain in the ongoing guessing game of who will eventually succeed Warren Buffettas Berkshire Hathaway's CEO.

Jain is in charge of Berkshire's profitable reinsurance operations. A Gen Re rep told CNBC that Tad Montross, CEO of the Berkshire insurance subsidiary, will retire by the end of the year.

When he does, the rep says, Montross' successor will report to Jain, a change from the current structure in which Montross reported directly to Buffett.

Jain has long been seen as among the top contenders to take over from Buffett once he's no longer running Berkshire.

Buffett regularly heaps a lot of praise on him, as he did in this year's letter to Berkshire shareholders:

"Ajit insures risks that no one else has the desire or the capital to take on. His operation combines capacity, speed, decisiveness and, most important, brains in a manner unique in the insurance business. Yet he never exposes Berkshire to risks that are inappropriate in relation to our resources."

Berkshire's energy chief, Greg Abel, is also seen as a top contender.

In his shareholder letter last year, Buffett wrote, "Both the board and I believe we now have the right person to succeed me as CEO — a successor ready to assume the job the day after I die or step down."

He hasn't however, revealed who that person is.

Buffett has said that he thinks his son Howard should succeed him as non-executive chairman of the board to "make change easier if the wrong CEO should ever be employed." Buffett doesn't think that's likely, but wrote, "I've seen how hard it is to replace a mediocre CEO if that person is also chairman."

By CNBC's Alex Crippen. Follow him on Twitter: @alexcrippen