The investigation, announced by the regulator, centres on whether Meta conducted suitable risk assessments for illegal content and risks to children before the feature was introduced in May.
Under the UK's Online Safety Act, platforms must carry out and update risk assessments to comply with duties including tackling illegal content and protecting children from harmful material such as pornography, content promoting self-harm or suicide, and abusive or bullying content.
Ofcom enforcement director George Lusty said the laws are clear: "Significant changes to platforms must be risk assessed before they're launched. This is a vital step that tech firms must take to protect their users." He added: "You wouldn't launch a new car without testing it properly, and it's the same for online services."
A Meta spokesperson said the company takes its regulatory obligations seriously. "We conducted a risk analysis and briefed Ofcom about this feature on a number of occasions before launching it," the spokesperson said. "We will of course co-operate with Ofcom during this investigation."
The probe will examine whether Meta breached duties by not carrying out "a suitable and sufficient illegal content risk assessment and children's risk assessment before the launch of Instants," according to Ofcom.