A CIS verification number is the reference code that HMRC issues to a contractor when that contractor verifies a subcontractor's status through the HMRC CIS online service (or by phone). It is the proof that the contractor completed the verification step and is applying the correct deduction rate as instructed by HMRC.
When a contractor verifies a subcontractor, HMRC returns one of three deduction rates (0%, 20% or 30%) along with the verification number. The contractor must:
- Record the verification number against the subcontractor's record.
- Include it on every payment and deduction statement issued to that subcontractor.
- Include it in the relevant monthly CIS300 return.
A verification number is not a one-off permanent reference. HMRC expects contractors to re-verify subcontractors before the first payment in a new tax year, and at any point where there is reason to believe the subcontractor's status may have changed (for example if they appear to have lost GPS). Since April 2026, re-verification before each payment is also one of the three due-diligence steps that protects a contractor against GPS revocation under Finance Act 2026's "knew or should have known" standard.
Practically, verification numbers follow a standard format (for example V followed by digits). They are generated automatically by the HMRC system and the contractor has no control over them. The key discipline is to record them accurately and not to make or continue payments without a current verification on file.
- Keep verification records for at least six years (the standard HMRC audit window).
- Re-verify whenever a subcontractor's circumstances appear to have changed.
- Never make a payment at 20% on the basis of a verbal assurance of GPS: verify first and get the number.
See how to verify subcontractors under CIS for the step-by-step process.