No explanation was given for the resignation of Ray Leonard, fuelling speculation it may have been linked to controversies surrounding GSOC, including its bugging investigation.

However, the Irish Examiner understands the resignation could be linked to a strongly-worded personal submission he sent to the Oireachtas justice committee last May.

Mr Leonard, who was acting director of investigations, was invited to give an oral presentation on the back of his written submission. He was to appear on May 14 last, but did not turn up.

This prompted speculation that his submission and scheduled public appearance created difficulties with the leadership of the three-person commission, headed by chairman Simon O’Brien.

In his written submission, Mr Leonard said the agency lacked “effective independence” and it did not represent “value for money”.

He also criticised the “climate” and “culture” in which “the GSOC chairman can be summoned by the minister to give an account of himself”.

This referred to the attendance of Mr O’Brien at the Department of Justice following a request by then minister Alan Shatter the day after The Sunday Times broke the bugging story.

Mr Leonard said the watchdog was “not an ombudsman in the ordinary meaning” and said it could not be independent as it was financially dependent on the Department of Justice.

It could not be confirmed yesterday if Mr Leonard was taking up a job elsewhere.

A GSOC spokeswoman yesterday said that a “senior officer” had tendered his resignation. She said the person was not one of the three commissioners, nor its director of investigations.

Sinn Féin’s justice spokesman Padraig Mac Lochlainn described the resignation as a “worrying development”.

He called on the ombudsman to state who had resigned and under what circumstances in order to clarify the situation and “not to allow further damage to GSOC’s reputation”.

The development follows dramatic calls last Friday from the Dublin South Central branch of the Garda Representative Association for the resignation of GSOC’s commissioners over its bugging investigation.

Mr Leonard was one of three GSOC investigators in that inquiry and was also acting director of investigations.

While the Cooke Report suggested GSOC was “premature” in suspecting gardaí, it said the investigating officers “acted in good faith”.