*Michael Jackson’s former physician Dr. Conrad Murray, currently serving a four year involuntary manslaughter sentence for causing his death, has threatened to release a bombshell secret, if forced to testify in the King of Pop’s wrongful death trial.

The cardiologist was subpoenaed to testify in Katherine Jackson’s ongoing $40 billion lawsuit against concert promoter AEG Live, and he has invoked his fifth amendment right protecting himself against self-incrimination, pending his upcoming appeal proceedings.

TMZ have obtained a voicemail message Murray left for a friend, in which he begs not to be put on the stand.

In the audio he can be heard saying, “Please stop. If you don’t, let this serve as my final warning to both sides (defense and prosecution). Don’t incite me to testify. Consider this as an imminent nuclear warning. Enough is enough.”

This is the second time Murray has verbally threatened to release a “nuclear” secret about Jackson, after leveling a similar demand in April. Both times the jailed doctor stopped short of revealing what information he has concerning the late pop star.

Katherine is suing AEG Live alleging they ignored signs her son was suffering from poor health and were negligent in hiring Murray to care for him preceding his ill-fated “This Is It” residency in London.

Executives at AEG Live maintain Jackson hired Murray as his personal doctor.