The health regulator removed Basildon and Thurrock from special measures in June 2014 after the trust improved the quality of its health services.
However, Monitor is concerned that the trust is now predicting a larger financial loss this year than previously thought.
Monitor’s investigation will seek to understand why Basildon and Thurrock’s finances have deteriorated and what can be done to fix them.
It will also examine how well the trust is working with other local NHS bodies to address its problems.
Laura Mills, Deputy Regional Director at Monitor, said:
Basildon and Thurrock has made great progress in improving its services for patients, but it appears to be struggling financially.
We want to find out why the trust’s finances are deteriorating and ensure its leadership is taking appropriate action to address this challenge.
Monitor will announce the outcome of its investigation, and whether any regulatory action is needed, in due course.