Tax and NI costs if found not to be self-employed.

I am starting as a self-employed consultant (sole trader) for technology commercialisation (business development). In a draft contract with a client, the client may "deduct PAYE income tax and NI Contributions from the consultancy fee" if HMRC determine that I am not self-employed but an employee of the client. It's a worst case scenario, but is there any possibility that the income tax and NIC could be higher than my actual fee? For the consultancy project, the sum planned is £5,000 inclusive of VAT. Thanks for any help.

Hi,
Unfortunately, it is not possible to review personal matters in this forum. For individualised assistance please contact our Self Assessment team for advice:  
Self Assessment: general enquiries
Thank you.