A council notice to parents explains that: "Safeguarding the children and young people who use the site is one of our top priorities.
"Due to Ofsted regulations we have a responsibility to ensure that every authorised adult who enters our site is properly vetted and given a Criminal Records Bureau (CRB) check by Watford Borough Council."
Council Mayor Dorothy Thornhill argued they are merely enforcing government policy at the play areas, in Vicarage Road and Leggatts Way.
She said: "Sadly, in today's climate, you can't have adults walking around unchecked in a children's playground and the adventure playground is not a meeting place for adults