UPDATE 1-Foundation to sell up to 3.55 pct Erste Group stake

* Foundation to sell up to 3.55% stake, keep about 20.2%

* CaixaBank boosts stake to just below 10%

* Erste reaffirms 2012 guidance on operating results

(Adds details and background)

VIENNA, Oct 1 (Reuters) - The foundation that is a major shareholder in Erste Group Bank will sell a stake of up to 3.55 percent in the Austrian lender, the bank and the foundation said on Monday.

"The stake in Erste Group controlled by Erste Foundation after the transaction will amount to an approximate 20.2 percent," a statement said, adding the foundation was not planning any further share sales.

Goldman Sachs International and Erste Group will act as joint bookrunners in the accelerated bookbuilt offering to institutional investors, it added.

"With this transaction Erste Foundation has concluded a share sale programme which the board ... had decided upon in spring 2012. The proceeds from the transaction cover the planned repayment of liabilities of Erste Foundation to its full extent," it said.

Erste said as well that Spain's CaixaBank , its second-largest shareholder, had purchased a block of one million Erste Group shares from the foundation, boosting its stake to slightly below 10 percent.

"Erste Group reaffirms its guidance for its operating result and risk provisions which it announced as part of its H1 2012 results presentation," it added.

(Reporting by Michael Shields; Editing by Mark Potter)

((Michael.Shields@thomsonreuters.com)(+43 1 531 12 258)(Reuters Messaging: michael.shields.thomsonreuters.com@reuters.net))