Speak or write first of all to the person who is responsible for your payroll or salaries department, setting out why you think you have been paid incorrectly.
If you don't get a satisfactory answer to your complaint you may wish to speak to your manager and, if you have one, with your union representative.
If you are not a member of a union then you might like to ask the assistance of a fellow colleague whom you trust to have a sound knowledge of the payment systems at your workplace.
If your complaint is not resolved through these informal means you may have to raise a grievance using your organisation's formal grievance procedure. If your employer fails to investigate your grievance or there remains a dispute, you could then consider whether to bring a claim to the Employment Tribunal (eg. For unlawful deduction of wages). Seek advice before going down this route.
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