The Los Angeles coroner is said to have overruled him and is due to investigate. Toxicology tests will be carried out on drugs found in the home, but results could take as long as six weeks.
Murphy is understood to have died as a result of cardiac arrest, the same cause of death recorded in the case of Michael Jackson in June.
Police are still investigating the singer's apparent dependency on prescription drugs. Coroner's spokesman Ed Winter confirmed prescription drugs were found at Murphy's home but added that crime was not suspected. He said her death "appears to be natural" and her family is co-operating with the probe.
However, a friend of the star was quoted as saying last night: "Brittany has been living life on the edge. She definitely had a drug problem and we have all begged her to get help. We have been very worried something like this was about to happen."
One unconfirmed report suggested that Murphy had struggled to kick prescription drugs since taking painkillers after cosmetic surgery. She is said to have struggled in the past with anorexia.
No illegal drugs were found in her home. She has denied ever taking cocaine, claiming in a 2005 interview: "My worst vice is caffeine."
Clair Staples, a neighbour, said she saw paramedics trying to revive Murphy as an apparently "dazed" Mr Monjack looked on. "She was fairly obviously dead at that time," said Ms Staples.
Murphy's mother Sharon, reportedly told doctors that her daughter was diabetic. The star was pronounced dead on arrival at hospital. Los Angeles Police confirmed that an investigation was launched. The actress's family released a statement saying: "She was our love and a shining star."
Actor Ashton Kutcher, who once dated Murphy and co-starred with her in 2003's Just Married, said on his Twitter page: "2day the world lost a little piece of sunshine."
