The regeneration specialist has been working on a £100m development at the Staffordshire garden and leisure site, which attracts 3m visitors a year, since buying it in 1996.
Trentham’s operations director is to be replaced by Mike Herbert, St Modwen’s North Staffordshire regional director, in a cost-cutting move.
Senior gardener Marcus Chilton-Jones has also left and will not be replaced.
A Trentham representative said: "All jobs are at risk".
St Modwens representative Charlotte McCarthy said:
Due to the current economic climate we are conducting a review at Trentham. Some aspects of the business are not as profitable as others. We’ve taken a view and restructured the management accordingly. The staff are under a consultation period but we expect the majority to be retained.
The 330ha estate will now be managed by St Modwen’s North Staffordshire Regional Office, currently based at Festival Park, Stoke-on-Trent, which will be combining with the Trentham management office and will move to Trentham.
St Modwen CEO Bill Oliver said: Some elements have not delivered the levels of income required to justify their running costs, this - coupled with the current difficult economic climate - means we have to take action to ensure that the future of Trentham is protected.
"We remain committed to Trentham and to maintaining its core features, particularly the beautiful Italian Gardens, park and lake.
Future projects including a London Eye-style big wheel, a second hotel and 75 new holiday lodges are still to go ahead.
McCarthy said the Blue Diamond garden centre on site should not be hit by the moves.
St Modwen lost £20m in the half year to July 2008. In an interim management statement for the three months to the end of September, the group said the market for residential land remains virtually non-existent and occupational property was also weakening. Job cuts will be confirmed in mid-November.