Eggert Magnusson has stepped down as chairman of West Ham and sold his stake in the club.
The club's owner Bjorgolfur Gudmundsson has become West Ham's club chairman as well executive chairman and has bought Magnusson's 5% shareholding.
Magnusson was part of the Icelandic consortium that bought the Hammers a year ago in a deal worth £108million with Gudmundsson, a banker and businessman, putting up most of the money.
Gudmundsson, who is investing a further £30.5million in the club, said: 'Eggert Magnusson has done a tremendous job as chairman, often in difficult circumstances, since our consortium bought the Club 12 months ago.
'Eggert has decided to move on to new pastures and to pursue his other interests. We wish him well for the future and express our thanks for all that he has done for West Ham.'
Magnusson admitted there had been tough challenges over the last year - the club was nearly relegated last season and the Carlos Tevez affair is still not completely over.
He said: 'I have thoroughly enjoyed my time here at West Ham. From the moment that Bjorgolfur Gudmundsson and I formed the consortium to bid for the club, it has been an honour to be involved.
'Now that I have sold my financial stake in the club and I need to focus a little more on my interests overseas, I think it is the right time to stand down as chairman.
'The last 12 months have not been without their challenges but I feel that we have emerged stronger and fitter as a club.'
Gudmundsson now wants to push ahead with a move to a new £250million, 60,000-seater stadium - a site has been identified at an old Parcelforce depot next to West Ham tube station.
Gudmundsson, who is now the sole owner of the club, added: 'We are seeking to build some truly solid foundations for the future ambitions of West Ham United FC. This is a great football club with tremendous heritage and tradition and the most wonderful set of supporters.
'My job as owner is to help plan for the future and create the right conditions off the field to help Alan Curbishley and the playing squad deliver success on the field.
'I am particularly delighted that we have been able to take the first real step in our plans for a new stadium, which is crucial to the long-term ambitions of the club.'