NIM39215 | National Insurance Numbers (NINOs): Confirming a NINO: Employee does not have a NINO
From HM Revenue & Customs · National Insurance Manual
If a new employee has never had a NINO, the employer should:
keep a record of their full name (surname and first name(s)), address, date of birth and gender, and
tell them to contact DWP on 0800 141 2075
For further details of the adult registration process, see https://www.gov.uk/apply-national-insurance-number
For details of special cases see NIM39600.