A state PCS officer who is already in service can appear for the UPSC Civil Services Examination. The rules are:

At the application stage:

• No formal No Objection Certificate (NOC) is required to apply
• The candidate must submit a written intimation to their Head of Department/Head of Office
• The UPSC application form requires an undertaking confirming this intimation has been given

Critical caveat: If the employer/department sends a communication to UPSC withholding permission, the application can be rejected or candidature cancelled. In practice, departments rarely withhold permission for competitive exams, but this risk exists.

At the Mains/Interview stage:

• Written intimation must be renewed at each stage
• For the Personality Test, the candidate generally needs formal permission or leave from the department
• Some states formally require an NOC at the interview stage

Appointment and resignation:

• If selected for UPSC CSE and allotted an All India Service or Central Service, the PCS officer must formally resign from state service before joining
• The resignation from PCS is effective from the date of joining the UPSC-selected post

Practical note: Many IAS officers today were previously state PCS officers who cleared UPSC CSE while in service. The process is well-established and routinely followed across states like Uttar Pradesh, Bihar, Madhya Pradesh, and Rajasthan.