UK cybersecurity agencies have launched an investigation into the London Stock Exchange (LSE) downtime in May 2019.

The Wall Street Journal reported that an inquiry by Government Communications Headquarters, commonly known as GCHQ, hopes to determine whether or not the August 16 incident was caused by a cyberattack. The UK Treasury is also said to be involved in the investigation.

At the time, the LSE described it as “a technical software issue” that impacted trading in FTSE 100 and 250 stocks, among others.

The incident delayed the day’s trading by more than 90 minutes, making it one of the worst stock exchange outages of all time.

GCHQ, the intelligence and security organisation responsible for providing signals intelligence and information assurance to the UK’s Government and armed forces, reportedly wants to determine whether hackers could have taken advantage of what was an IT system update at the time to disrupt markets.

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The LSE stands by its assertion that the incident was the result of a software configuration issue following an upgrade.

A spokesperson for LSE said: “London Stock Exchange takes its commitment to run orderly markets for its members seriously and has thoroughly investigated the root cause of the issue to mitigate against any future incidents,” a spokesperson told the paper.

A spokesperson for the National Cyber Security Centre, part of GCHQ, said: “The NCSC has not treated the LSE outage as a cyber security related incident and has not investigated it as such.”