"This will allow us all to concentrate on next season and beyond, with Gianfranco Zola, technical director Gianluca Nani and I working to build on what we have achieved. We are all committed to a long-term vision."
Hansa, the holding company for the Hammerd, had until Monday 8 June to sell the club in order to prevent creditors seizing its assets and forcing the sale of West Ham.
CB Holding was set up last week by a selection of the club's major creditors.
Gudmundsson, who lost heavily in the credit crunch, commented: "I want to thank everybody at Upton Park for unforgettable years. As my fortunes have changed I now have to withdraw from the board of directors.
"I do that with great regret but I am convinced this change of ownership and control of West Ham will advance the club.
Upson welcomes West Ham takeover
"I will remain forever a West Ham fan and hope I will have many returns to West Ham."
The Hammers finished ninth in the Premier League last season and defender Matthew Upson said the takeover was crucial to the club's hopes of continuing to improve.
"The club is in a really crucial position because we had a positive season and it would be really nice to build on that," said Upson, who joined the club from Birmingham in January 2007.
"I am very positive that the club has got some good financial investment, and hopefully we can go on and strengthen the squad.
"I want to improve as a player and see the club develop. My future is there, I am very happy enjoying my football under Zola, so the times are good."
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