Sullivan and co-chairman David Gold had been equal partners since taking over the Barclays Premier League club in January 2010.
But Sullivan recently purchased an additional 25 per cent share from CB Holding, the firm which had controlled West Ham on behalf of Icelandic banks after former chairman Bjorgolfur Gudmundsson was declared bankrupt.
Sullivan's investment is part of a £25.5million restructuring of the club's debt.
The club's official website shows that Gold still owns 30.6 per cent of WH Holding Limited, the club's parent company, and CB Holding's stake has been reduced to 10 per cent.
- Investment & Company Information
- Bjorgolfur Gudmundsson