An MOJ spokesperson said: “We are appalled that this happened and recognise the distress it will have caused victims, survivors, and their families.
“We apologise to those affected – unauthorised access to court files is completely unacceptable.
“This is now being investigated urgently, and the prime minister has asked the Lord Chancellor to oversee this.
“All wrongdoing will be met with extremely firm action.”
The department said it was also reviewing policies and frameworks covering staff conduct, acceptable use and its fraud response plan.
HM Courts and Tribunals Service and HM Prison and Probation Service are also investigating.
Bebe King, six, Elsie Dot Stancombe, seven, and Alice da Silva Aguiar, nine, were murdered during the attack at a Taylor Swift-themed dance class in Southport on 29 July 2024.
Ten others were physically injured.
The data breach disclosure follows separate cases involving inappropriate or potentially inappropriate access to the medical records of people involved in the attack.
In May, a Liverpool hospital trust admitted nearly 50 staff had inappropriately accessed the records of some victims treated at Aintree Hospital.
North West Ambulance Service later began investigating potential inappropriate access to patient records by some of its staff.
