Hackers breached the network of one of the largest clinical laboratories in America sparking fears of a major cyber security breach, DailyMail.com can reveal.

According to a company insider senior managers were informed that the entire computer network of LabCorp, a Fortune 500 company, was shut down across the US Sunday morning after hackers tried to access the private medical records of millions of people.

The firm's tech experts are now working to bring the system back online after what a LabCorp spokesperson told DailyMail.com was 'suspicious activity'.

But while the firm insists there is 'no evidence' of 'unauthorized transfer or misuse of data', a company insider claims it could be weeks before LabCorp's experts discover the extent of the breach and whether or not the hackers were 'pulling data'.

According to a company insider senior managers were informed that the entire computer network of LabCorp, a Fortune 500 company, was shut down across the US Sunday morning after hackers tried to access the private medical records of millions of people.

The insider said: 'The only reason for a nationwide shutdown would be in a scenario where there was suspicion of a data intrusion.

'LabCorp was hacked and the suspicion is they were pulling data but he full extent of what was accessed if anything isn't clear.

'The company acted swiftly to stop the intrusion, but the fear is the private medical information of millions of patients may have been accessed.'

LabCorp runs a vast network of labs and patient centers nationwide performing routine and specialty diagnostic testing; the most common tests being bloodwork, urine analysis, and HIV tests.

The firm's headquarters is in Burlington, North Carolina and it also operates the National Genetics Institute in Los Angeles, with a wider network of 36 primary labs across the country.

LabCorp, which has a $10 billion plus annual turnover, also provides testing in Puerto Rico and outside the US, in three Canadian provinces.

On its website LabCorp makes a promise to protect the private medical information of its patients.

It states: 'LabCorp is committed to the protection of your PHI (protected health information) and will make reasonable efforts to ensure the confidentiality of your PHI... We take this commitment seriously and will work with you to comply with your right to receive certain information under HIPAA.'

The firm also states it is required to notify patients of any data breach within 60 days after discovery of the breach.

LabCorp runs a vast network of labs and patient centers nationwide performing routine and specialty diagnostic testing; the most common tests being bloodwork, urine analysis, and HIV tests

Kevin Gunter, Global Security Senior Manager for LabCorp declined to comment on the breach when contacted by DailyMail.com.

In a statement from LabCorp, the company said: 'During the weekend of July 14, 2018, LabCorp detected suspicious activity on its information technology network.

'LabCorp immediately took certain systems offline as part of its comprehensive response to contain the activity.

'This temporarily affected test processing and customer access to test results on or over the weekend.

'Work has been ongoing to restore full system functionality as quickly as possible, testing operations have substantially resumed today, and we anticipate that additional systems and functions will be restored through the next several days.'

The firm said customers of LabCorp Diagnostics may experience brief delays in receiving results.

'The suspicious activity has been detected only on LabCorp Diagnostics systems,' it added.

'There is no indication that it affected systems used by Covance Drug Development.

'At this time, there is no evidence of unauthorized transfer or misuse of data.

'LabCorp has notified the relevant authorities of the suspicious activity and will cooperate in any investigation.'

Last year there were 477 healthcare data breaches reported to the US Department of Health and Human Services (HHS) or the media, affecting more than 5.5 million patient records, according to Protenus, which tracks disclosed breaches impacting the healthcare industry.