GCHQ, the government’s electronic eavesdropping agency, has been found to be spying too much on its own staff, an official watchdog has said.

Sir Mark Waller, the intelligence services commissioner, reveals in his annual report that GCHQ reported one error to him last year when it was found that an internal monitoring system, which keeps an eye on staff communications, was capturing an unauthorised amount of information.

“I followed up on this error during my May inspection, and the team explained that because of a lack of understanding of the systems’ full capability more data than had been authorised had been collected,” said Waller.

“It was clear to me this was a technical error and not deliberate. Following the discovery of the error, GCHQ deleted the captured data and reconfigured the system to ensure that it only collected the information it was authorised to collect. I continue to monitor this project to ensure that this error does not happen again.”

The GCHQ episode is among 43 errors by the security agencies, reported in 2014 by the intelligence services commissioner, which led to intrusion of personal privacy to some degree.

The commissioner revealed that, when he first started monitoring the misuse of data, there were two serious data breaches by individual security service contractors, each of whom had carried out “unnecessary queries” of bulk datasets (the mass of highly personal information held by the intelligence services on named individuals) without proper “business” justification.

Waller said: “Both were contractors, and in both cases, following investigation, they were escorted from the premises and their contract revoked.”

More recently, an MI6 officer was issued with a breach notice after he accessed the system containing the bulk personal datasets after moving to a job that did not require access. He was reprimanded, even though his access to the highly personal information was for a legitimate work purpose. The commissioner said that such unacceptable uses were few in number.