Seeking Guidance on Resignation and Relieving Letter After Deloitte Pre-Hire Questionnaire

I have cleared the interview rounds and recently received an email from Deloitte regarding the Pre-Hire Questionnaire. As part of the process, I am required to submit my relieving letter from the current organization.

However, I have not yet received a formal offer letter from Deloitte. This makes me a bit hesitant to submit my resignation. Has anyone else been in a similar situation?

Additionally, if I proceed with the resignation, how many days does it typically take to receive the relieving letter from TCS?

Any insights or experiences shared would be greatly appreciated.