Employement problem related to negative feedback shared by ex employer

I recently joined an organization in BPO sector in NAvi Mumbai and after working for few months, my current employer came back saying I have to look out for new job as the ex employer where I worked 5 years back has mentioned I was removed from the company because of integrity issue. However, when I left the earlier organization I resigned and my resignation was accepted and i have relieving letter and full and final statement. New Employer is pressurizing me to resign. What can I do to protect my job in current organization and also what can I do to my ex employer who is tarnishing image.