Sky’s report included a claim from one of the criminals that the fraudulent certificates were being processed by “an insider” at TfL.
But the organisation confirmed on Friday that, having reviewed its processes, it had found no evidence of any fraudulent activity by TfL employees.
The criminals were said to be placing adverts on Facebook Marketplace, TikTok, Snapchat, Instagram and WhatsApp, sometimes charging up to £1,000 to purportedly make any car appear Ulez-compliant to TfL, allowing its driver to dodge the daily £12.50 charge.
The adverts claim to be able to perform the trick in as little as 24 hours, using nothing more than the vehicle's logbook.
One seller told an undercover Sky reporter they had successfully “processed” more than 100 vehicles using the method.
A TfL spokesman said: “We are proactive in identifying criminals attempting to commit fraud against TfL and are aware of a small number of criminals attempting to produce fraudulent counterfeit vehicle documents to support fraudulent ULEZ exemption applications.
"We are working with the police to make sure the criminals face the consequences of their actions and are working closely with vehicle manufacturers to prevent fraudulent applications being successful.”
He added: “Drivers should not use the services advertised by these fraudsters as they will also be committing a criminal offence as well as wasting their money as their exemption will not stand. We have already removed many exemptions where we have identified or suspected them to be fraudulent.”
The Ulez was expanded by mayor Sadiq Khan to cover the whole of Greater London on Tuesday, bringing roughly five million more Londoners into its territory. It used to only extend as far as the North and South Circular roads.
Mr Khan has previously warned non-compliant vehicle drivers not to be taken in by unofficial websites claiming to offer easier ways of paying the £12.50 charge.
TfL said that any “additional services” those payment websites claim to offer “are either non-existent or are already provided by TfL without charge”.