Earlier this month, he found that Dr Peter Prinsley, a Labour MP, had inadvertently breached the rule by discussing a standards investigation with a BBC journalist.
The commissioner said the MP had breached paragraph 13 of the code of conduct by correcting the journalist who had asked a question about an ongoing investigation based on incorrect information.
The breach was deemed to be “inadvertent” because Dr Prinsley had been caught unawares by the question and had no intention of influencing the investigation.
‘Talked about it in pre-scripted speech’
A Labour source said: “Nigel Farage’s disclosure of the widening of the investigation cannot be said to be inadvertent.
“Farage talked about it in a pre-scripted speech, and more details appear to have been subsequently briefed to The Guardian by people with knowledge of the situation.”
The source added that, were Mr Farage to be re-elected on Aug 13, “the commissioner will also have to investigate whether he broke the code of conduct by disclosing information about the investigation without authorisation”.
A senior Whitehall source said they believed the Reform leader had committed a “technical breach”, which could lengthen any penalty he is given were the original inquiry into undeclared donations to find against him.
Mr Farage faces potential sanctions ranging from a formal apology to a parliamentary suspension. A suspension of 10 or more days would allow his constituents to trigger a recall petition forcing a second by-election in Clacton.
The inquiry is not expected to conclude until September at the earliest.