The West London Mental Health NHS Trust said it had called in independent investigators to look at historic management of finances relating to capital projects.
It is reported that the capital estates department has overspent by £4m on unfunded or unapproved projects.
A spokeswoman for WLMT said: “The trust has commissioned an independent investigation into the historic management of finances relating to capital projects undertaken within the capital estates and facilities department of the trust.
“We are unable to comment in detail on the investigation while it is ongoing.
“However, we can assure the public that the trust takes this matter very seriously and the investigation will be full and impartial, and will interview all those involved in the management of the department at that time.
“The trust board will take any appropriate actions as recommended in the final report.”