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HMRC internal manual

National Insurance Manual

Class 1 Structural Overview: More than one employment with the same or different employers

Paragraph 1 of Schedule 1, SSCBA 1992

In general, all earnings from employed earner’s employment(s) with the same employer must be aggregated before determining the amount of NICs due.  Additionally, under certain prescribed circumstances, earnings from employed earner’s employment with different employers must also be aggregated.

Further detailed guidance is contained within NIM10000 onwards.