Health watchdog Monitor will announce a probe into the region's mental health services on Thursday morning, amid financial concerns.
The Norfolk and Suffolk Foundation Trust will be investigated over whether it has breached its licence by racking up a deficit of close to £2 million this financial year.
The trust is under severe financial pressure and made cuts of £15m from its budget of £200m last year.
It was aiming to break even in this financial year, but the deficit could rise to more than £4m by the end of the financial year in March 2015.
The overspend is mainly down to the cost of hiring temporary agency staff and sending patients out of Norfolk and Suffolk for treatment.
The organisation is looking to reduce its budget by 20pc between 2012 and 2016 as part of an NHS efficiency drive.
And it spent £16.3m on temporary staffing costs during 2013/14, according to its annual report.
NSFT chief executive Michael Scott has argued for more money for mental health services.
He said: 'We are all well aware that the NHS both locally and nationally is under intense pressure both in terms of demand for services and funding and we too are facing considerable challenges.
'There are particular funding problems facing us in mental health and we welcome Monitor's review.
'In the interests of our service users and staff, we are determined to see a fairer deal for mental health and we're working to improve the culture and leadership of the trust. We look forward to working with our regulator and commissioners to make sure we get back on track financially.'
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