Medibank said it received messages from a group wishing to negotiate with the provider “regarding their alleged removal of customer data”.

“This is a new development and Medibank understands this news will cause concerns for customers and the protection of their data remains our priority,” Medibank said in a statement.

“Medibank is working urgently to establish if the claim is true, although based on our ongoing forensic investigation we are treating the matter seriously at this time.”

Medibank remains operational and said no ransomware attacks have happened yet.

Today the health insurer halted trading on the Australian Stock Exchange as a result of the threat.

“Medibank has entered into a trading halt, to ensure that it meets its continuous disclosure obligations. The trading halt will continue until further notice,” Medibank said.

Medibank shares last traded at $3.50.

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It is the second time in a week the provider has been forced to stop trading.

The insurer added it is working with cyber security firms and the Australian Cyber Security Centre to get to the bottom of the cyber incident.

“I apologise and understand this latest distressing update will concern our customers,” CEO David Koczkar said.

“We have always said that we will prioritise responding to this matter as transparently as possible.

“Our team has been working around the clock since we first discovered the unusual activity on our systems, and we will not stop doing that now.

“We will continue to take decisive action to protect Medibank customers, our people and other stakeholders.”

No such reassurance was provided by the health insurer to customers today.