An undercover police officer has admitted he was exposed as an infiltrator by his own blunder, which has been described by activists as worthy of Inspector Clouseau, the spycops public inquiry has heard.

The officer, who used the fake name Simon Wellings, jeopardised his own covert deployment by mistakenly recording himself discussing individual campaigners with other special branch officers.

He committed the error while infiltrating leftwing groups as a member of a covert Scotland Yard unit.

It happened when he attended a secret meeting with the special branch officers and was asked if he had the telephone number of an activist. He looked up her number on his phone, but then mistakenly dialled it.

His call was diverted to the activist’s voicemail, which recorded him being asked by the other officers to identify campaigners from photographs he was being shown. He did not realise their conversation was being recorded.

Internal police documents show that, after he was exposed, the police considered whether to leave the anti-capitalist group he had infiltrated “intact” or whether to “mount a destructive operation”.

Wellings was questioned last week by the inquiry that is examining how about 139 undercover officers spied on tens of thousands of predominantly leftwing activists over the course of more than four decades.

The undercover officers assumed fake identities as they hoovered up information about the political and personal lives of the activists.

Wellings said he submitted up to 4,000 surveillance reports on campaigners while he infiltrated Globalise Resistance, the anti-capitalist group, and other leftwing campaigns between 2001 and 2007. These included details of campaigners’ bank accounts, housing, personal relationships and finances.

Wellings made the accidental recording in 2004 while meeting with officers in special branch, the secretive police division that monitored political groups.

Activists from Globalise Resistance recognised his voice from the recording. They suspected Wellings was an informant and confronted him at a meeting at the Royal Festival Hall, London. Wellings was expelled from the group.

Guy Taylor, the group’s national organiser, said Wellings’ mistake was reminiscent of Inspector Clouseau, the fictional detective who was famously inept.

Four days after the expulsion, the episode was recorded in a memo by DCI Michael Dell, the then head of the covert unit, the Special Demonstration Squad (SDS). Setting out next steps, Dell wrote: “Consider with [special branch] whether we should leave Globalise Resistance intact or through [Wellings] mount a destructive operation.”

Asked by the inquiry what this meant, Wellings said: “I have no idea … I don’t really understand what ‘mount a destructive operation’ means, or how we would have the agency to do it even if it was remotely appropriate or lawful.”

Dell is likely to be questioned later this year at the inquiry, which has yet to hear any further evidence about this memo.

Wellings said he “held himself responsible” for compromising his deployment. After his expulsion, Wellings switched to monitoring other leftwing activists for three more years.

Campaigners said Wellings routinely made up and over-inflated his surveillance reports about them, exaggerating, for example, the level of violence in protests.

Wellings rejected the criticism, saying: “I did my best to report on what I saw and heard and experienced around myself.”

Many of his surveillance reports covering the last years of his deployment consisted of information taken from campaigns’ websites. Wellings said one of the reasons he joined the SDS was the significant amount of overtime that he claimed each month.