Parents in England who regularly look after friends' children and receive a "reward" for doing so must register as childminders, regulator Ofsted says.
It said most parents would be exempt but those who babysat for more than two hours at a time, or for more than 14 days per year, should be registered.
The "reward" could be money or free baby-sitting in return, it said.
The warning comes after Ofsted told two policewomen to end an arrangement to care for each other's children.
According to the Mail on Sunday, Ofsted told two detective constables, Leanne Shepherd, from Milton Keynes, and Lucy Jarrett, from Buckingham, to end their arrangement.
