Almost 1,000 care homes across England have no registered manager, a watchdog said.
While there is no direct risk to residents, care homes without a registered head may be less able to spot problems, the Care Quality Commission (CQC) said.
The new Health and Social Care Act 2008, which came into force last week, makes it a requirement for all care homes to have a registered manager. The old legislation, brought in in 2000, also carried the same stipulation.
Yet, despite this, almost 1,000 care homes have no registered manager.
The CQC said it had placed conditions on those care homes without a registered qualified manager, requiring operators to appoint one.
There are more than 500 different operators running the homes and they must appoint a registered manager by April 1.
If they fail to do so, they will be breaking the law.
Some other care homes have a registered provider who manage their own services on a day-to-day basis, and they do not also need to register as managers.
Another 200 care home providers have had conditions placed on their registration for other "compliance issues," the CQC said.
Chief executive, Cynthia Bower, said: "It has been recognised in the care sector for some time that there is a shortage of experienced and qualified managers. While we have been undertaking the enormous task of re-registering thousands of care services, the scale of this has become clearer."