Two police officers have been placed under criminal investigation over the shooting of a robbery suspect in south-west London, the Guardian has learned.

The firearms officers each fired a single shot as Scotland Yard’s flying squad staged a planned operation to thwart a heist in Wimbledon in December last year. A 23-year-old man was wounded in the incident.

Searches of the scene by investigators found no sign of a firearm, said the Independent Office of Police Conduct (IOPC), which is investigating the case. Some media reports quoted witnesses claiming police had been fired upon.

The watchdog has decided the officers’ actions in opening fire could amount to a criminal offence as serious as grievous bodily harm (GBH). The officers were interviewed under criminal caution in December and have not been arrested.

The IOPC said in a statement: “We can confirm that the two Metropolitan police officers who discharged their weapons during the police shooting in Wimbledon in December have now been interviewed under criminal caution for GBH and health and safety at work offences as part of our ongoing investigation.

“An interview under criminal caution means there is an indication a criminal offence may have been committed. It does not necessarily mean that criminal charges will follow.” Evidence gathered to date is being examined and various lines of enquiry are continuing. It would not be appropriate to comment further.”

Police firearms officers can open fire if they have an honest belief there is a threat to their own life or those of others. Firearms officers volunteer for the role and police chiefs say those chosen to carry firearms are highly trained.

The presumption of the police watchdog is to treat officers who shoot suspects as witnesses unless material emerges to suggest otherwise. The decision to treat these two officers as suspects is likely to anger firearms officers.

Last week the home secretary, Sajid Javid, signed off on revised rules that firearms officers say do not offer them enough protection and could subject them to years of investigation and stress for doing their job to protect the public.

The shooting happened at about 8.10am on Monday 3 December in Lacock Close, Wimbledon. After the incident the Met said two suspects had been charged with conspiracy to commit robbery. One, identified by the Met as Brooklyn McFarlane, 23, from Wandsworth, south London, was first taken to hospital and was charged after he was discharged.

The IOPC said at the time: “We understand that the shooting occurred during a pre-planned operation carried out by officers from the [Met’s] serious crime directorate, assisted by firearms officers, into suspected cash-in-transit-van robberies.

“So far a knife and a crowbar have been recovered from the scene and the forensic search of the area resumed today. We have verified that two MPS firearms officers each discharged their weapon once. The man was taken to hospital; his injuries are not believed to be serious.” A stolen car was also allegedly found at the scene.

The Met said both officers had been placed on restricted duties and did not make any further comment.