The Ministry of Justice knew its court computer systems were “obsolete” and “out of support” long before the network went into meltdown last week, internal documents have revealed.

An MoJ digital and technology review warned last year that judges in employment tribunals were threatening to stop hearing cases because of the unreliability of communications and 30,000 users were on systems that needed “immediate remediation”.

The detailed assessment seen by the Guardian blames “historical under-investment in ageing IT systems” and warns that it is reaching “unacceptable levels” of risk which could result in large-scale data breaches.

Courts across England and Wales have been disrupted over the past two weeks because of successive IT failures which began when the criminal justice secure email system went down, depriving 75,000 lawyers and staff of communications.

The MoJ’s main computer network linking lawyers, judges, probation workers and court staff also malfunctioned. Cases were delayed, access to the courts’ digital case system was denied and jurors could not be enrolled.

The justice minister Lucy Frazer was forced to answer emergency questions in the Commons last Wednesday during which she said there had not been a cyber-attack and also denied the problems were the result of financial cuts.

The internal review, however, makes it clear that long-term underfunding is at the heart of computer weaknesses in the MoJ. The department, which has to pay for courts, prisons and probation, has suffered larger cuts than any other in Whitehall since 2010. Its current funding level is almost 40% lower than it was in 2010.

The MoJ document, entitled Digital & Technology, says: “Historical under-investment in ageing IT systems has built our technical debt to unacceptable levels and we are carrying significant risk that will result in a large-scale data breach if the vulnerabilities are exploited.”

It adds: “We have a Technology 2022 strategy, but it is not funded to help us address the long-term issues with current systems and allow us to make best use of new technologies to improve service delivery.”

In one case study it refers to a database used by 16 employment tribunal administrative offices in which the “scale of outage” accounted for 33% of incidents over the previous six months. Users were unable to access systems for a “significant number of hours”.

The report cites problems such as “risk of database corrupted leading to data loss; unable to restore service in a timely manner”, and adds: “Judges say they will put tribunal activity on hold because of the poor running of the application.”

Addressing what are said to be “secondary risks”, the document notes: “Failure to reduce and prevent technical debt leading to a continued reliance on obsolete and out-of-support systems that may lead to operational failure and cyber-breach.”

The document also contains a series of colour-coded levels relating to “criticality”, ranging from the worst, black, to the least dangerous, green. Among those in the black category were 30,000 users whose systems required “immediate remediation”.

The MoJ said the document was a routine risk assessment of its computer systems. The case study referred to in the document is understood to relate to a different system to the one that broke down last week.

A spokesperson said: “Like any responsible organisation, we carry out regular assessments of risks to ensure that we can identify and manage them.

“It would be entirely wrong to suggest that last week’s IT issues were caused by us ignoring a risk assessment. We have always been clear that there are a number of outdated IT systems that need replacing which is why we continuously review our IT infrastructure and invest accordingly.”

Commenting on last week’s courtroom IT chaos, Chris Henley QC, the chair of the Criminal Bar Association, wrote in a message to members: “Whatever the reasons, and whatever the fix, has anything been more emblematic of the deep and worsening crisis in the criminal justice system than the collapse of IT systems in courts across the country last week?

“Trials adjourned, evidence inaccessible, secure emails vanishing, wifi down, Xhibit just a blank screen, prisons beyond communication, the stuff of satire, and a high-profile media story for several days … But it’s OK, it had nothing to do with a chronic lack of resources.”