The co-operation and competition panel is investigating the takeover of a trust that was not going to succeed in becoming a foundation.
The entire non-executive team of Bedfordshire and Luton mental health and social care partnership trust announced their decision to stand down in February after deciding it was not going to make foundation trust status.
Out of the 21 NHS organisations that expressed an interest in the acquisition, four have been shortlisted.
They are Hertfordshire partnership foundation trust, South Essex partnership university foundation trust, Camden and Islington foundation trust and Cambridgeshire and Peterborough foundation trust.
The co-operation and competition panel will assess whether the changes would have an adverse effect on patients or taxpayers and the extent to which this could limit competition for mental health services contracts in Bedfordshire and Luton or beyond.
It wil also weigh up any benefits to patients or taxpayers.
NHS East of England director of strategy Stephen Dunn said: “This process is a first for the NHS. The quality of the proposals made by those who initially expressed an interest, and especially for those who have got through the first stage, was very high and we are pleased that whoever ends up running the services in Bedfordshire and Luton will have patients and staff front and centre in their proposals.”
The co-operation and competition panel welcomes any submissions from interested parties submitted by 24 July.