Contractors working in the public sector through a personal service company (PSC), such as a limited company, are no longer able to determine their own IR35 status as of 6 April 2017.
HMRC passed legislation for off-payroll workers in the public sector to stop the tax avoidance of workers engaged through their own limited company, who would otherwise be working as a regular employee if not for their PSC arrangement.
If you work in the public sector through your own limited company, your public sector client is responsible for determining your IR35 status, and should take reasonable care in making the assessment.
If your client deems your assignment as being inside IR35, they must inform your recruitment agency or other third party body, who will then make the necessary tax and National Insurance deductions before making payment to your limited company. If you contract directly through the public sector body, the public sector body will make the deductions.