TOKYO (AFP) - Embattled Tokyo Electric Power, the operator of Japan's crippled Fukushima nuclear plant, has been slammed by an independent auditing watchdog for skimping on its computer network.

Facing multi-billion dollar cleanup and compensation bills, TEPCO figured it could save a few yen by delaying an upgrade to its outdated Windows XP operating system.

But an independent auditing watchdog - which usually monitors wasteful tax spending - has warned the vast company over its frugality, saying it must replace the system due to security concerns.

The utility - which was effectively nationalised through a massive government bailout after the 2011 atomic crisis - was hoping to save 3.6 billion yen (S$40.7 million) by continuing to run about 48,000 computers company-wide on the old operating system until 2018.

Microsoft stopped issuing security updates and technical support for Windows XP last year, aggravating concerns about cybersecurity.

"The company decided, on its own initiative, to move up the deadline to update the software due to system security concerns," a TEPCO spokesman said Tuesday.

In a report last month, Japan's Board of Audit warned the company not to be so stingy.

"Upgrading the operating system must be done as swiftly as possible, and the firm must not push it back given the security risks," the board said.