GLOBAL - Charles Prince, the chairman and chief executive officer (CEO) of America's largest bank Citigroup, has retired on the back of an estimated additional $11bn (€8bn) loss at the firm, directly-linked to the US sub-prime crisis.

Citi's board of directors today announced Robert Rubin, former US treasury secretary, chairman of Citi's executive committee and a member of the board of directors, will serve as chairman in the interim.

German-born Sir Win Bischoff, the former Schroder chairman, will take over for now as chief executive, but the board has tasked a committee, headed by Rubin, with finding a new CEO.

In a statement released on Sunday, Citi said it had suffered "significant declines since September 30 in the fair value of the approximately $55bn in US sub-prime -related direct exposures in its securities and banking business."

Citi has already announced a $6.5bn write-down last month for loan losses, sub-prime mortgages and other debt.

The group estimated in Sunday's statement, at the present time, the reduction in revenues ranges from approximately $8bn and $11bn, and is likely to impact the group's fourth quarter results.

Citi added it expects its capital ratios "will return within the range of targeted levels" by the end of the second quarter of 2008.

There is further speculation today Citi's losses might result in job cuts in its London branch.

Media reports also suggest Citigroup is looking to launch a cost-cutting initiative in its new institutional clients group, partly to free up resources to reward top talent, and boost shareholder returns.

Officials at the company were unavailable for comment at the time of going to press.