Detentions under the Mental Health Act have jumped 47% in the last decade, figures show.
In 2015/16, there were 63,622 detentions in England, up 9% on the previous year.
The figure has risen by 47% since 2005/06, when there were 43,361 detentions under the act.
The NHS Digital data also showed that more people are being taken to hospital as a place of safety, as opposed to police cells.
Some 22,965 detentions involved hospitals, an 18% rise on the previous year.
Dr Paul Lelliott, deputy chief inspector of hospitals at the Care Quality Commission (CQC), said: "It is concerning to see that more people are being detained under the Mental Health Act than in previous years, when there is a national commitment to reduce this number.
"The causes of the rise in the use of the Act are likely to be complex, but the increase in detentions needs to be examined carefully.
"We do not know, for example, the extent to which the rise is due to repeated detentions.
"It could signal a lack of support in the community for people with serious mental health problems or, if people are being detained repeatedly, it could be a sign that some services are operating 'revolving door' admissions.
"To get to the bottom of this, we are launching an investigation into the reasons why detentions under the Mental Health Act in England continue to rise. We expect to publish our findings from this next year."