FRANKFURT, May 26 (Reuters) - The supervisory board of Deutsche Bank backed its embattled chairman again on Tuesday amid claims that he lost what insiders describe as a power struggle with Chief Executive Josef Ackermann.

Sources familiar with the matter had said Chairman Clemens Boersig had pushed aside Ackermann's preferred successor last month and made an unsuccessful bid for the chief executive's job himself.

But they said the group's supervisory board threw out his offer. Instead Ackermann postponed his planned retirement, keeping the job for himself until 2013.

Reports of a row between the two have dominated German media for weeks, raising the spectre of a management split at the top of the country's biggest lender.

But on Tuesday, the group's supervisory board attempted to throw cold water on claims that Boersig had made a failed bid for the top job.

"The supervisory board considers the criticism of Dr. Boersig...to be completely unfounded," Tilman Todenhoefer, deputy head of the supervisory board, told the group's annual shareholder meeting.

"He did not pursue personal objectives at any time."

Separately, Ackermann said the bank was bracing itself for a difficult year as the global economy struggles to emerge from recession.

Ackermann, who initially described the crisis as over for his bank before it had really begun, has chalked up more than 10 billion euros ($20.5 billion) of writedowns as earnings from many of its businesses shrivel.