The investigation was passed onto the Competition and Markets Authority (CMA) by energy regulator Ofgem after it found employees staff and representatives had previously expressed concerns to some of the parties under investigation.
The CMA expects to make a decision on whether to proceed with the investigation in August, according to a statement posted on its website today.
"The CMA has not reached a view as to whether there is sufficient evidence of an infringement of competition law for it to issue a statement of objections to any of the parties under investigation," it said.
It continued, "Not all cases result in the CMA issuing a statement of objections."
"As a result, it would not be appropriate at present to include any further estimates of the timing of any later investigative steps at this stage."
Ofgem added: "Ofgem became aware of communications between Ofgem staff and representatives of some of the parties under investigation encouraging those parties to change their behaviour in relation to bidding on search advertising keywords relevant to Ofgem."
"Ofgem considers that this could delay the progress of the case, for example if parties were to call into question Ofgem’s impartiality in continuing with the case."
"The CMA has the same powers to investigate this case as Ofgem, so in the circumstances we therefore considered the CMA was better placed to continue with the investigation."