This part of GOV.UK is being rebuilt – find out what beta means

HMRC internal manual

National Insurance Manual

Special Cases: Married Women: Conditions that apply to certificates of election/reduced liability

Regulation 133, SS(C)R 2001

A married woman can apply for a certificate whether or not she is working at the time.  If she has more than one employer, she must apply for extra certificates.  A certificate belongs to the Department, but the woman is responsible for its safe-keeping until she gives it to her employer or sends it back to the Department.  She must give the certificate to her employer without delay and must tell them immediately if her right to reduced liability ends.

When the employer has the certificate, they are responsible for its safe-keeping until they give it back to the woman or send it to the Department.