5 Amendment of the Income Tax (Pay As You Earn) Regulations 2003
In regulation 15 (flat rate codes), before paragraph (1) insert—

“A1 HMRC may determine that the code for use by an employer in respect of an employee for a tax year is the additional rate code, if they have reason to believe that the employee will be chargeable at the additional rate on all or a substantial part of the employee’s relevant payments.”

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