The UK's second biggest advertiser, COI Communications, is entering talks with Kelvin MacKenzie about the possibility of planning its radio advertising based on Rajar rival GfK Media's measurement system.
The decision by the COI, which spent more than £23m on radio billings between November 2002 and October 2003, piles more pressure on Rajar, which has come increasingly under fire for failing to switch from a diary to an electronic measurement system.
MacKenzie, chairman and chief executive of The Wireless Group, has been funding the GfK research and has been fighting a long-running battle with the radio industry body over its reluctance to bring in electronic measurement.
In the past week, some major advertisers have added their weight to the calls for change.
Word that COI is looking at GfK's measurement system comes a week after Transport for London revealed to Media Week that it was considering rethinking its entire £1m radio budget on the basis of GfK's findings - which show a much higher reach for talk radio stations, such as MacKenzie's TalkSport, than Rajar's diary system.
But even TfL's radio budget is dwarfed by that of the COI, which issued a statement saying it would be meeting McKenzie in the next few weeks.
"Kelvin MacKenzie has made a strong case for changing the diary system," it said. "We would support using the latest technology to improve on the current system. We see this as an industry-wide issue.
"We are interested in what TfL is doing and will be following it very closely," it added. "We are under constant scrutiny when it comes to accountability."
GfK Media director Nick North welcomed the move. "I don't think you can underestimate the significance of this."
This week TfL and its media agency, PHD, exclusively revealed details of the major research project they are carrying out to compare the two systems as a media planning tool.
TfL and PHD said their study, which pits the effectiveness of GfK and Rajar head to head, involves a survey of 3,500 radio listeners during a forthcoming £500,000 road safety campaign on Capital FM, Heart, Magic, LBC, Classic FM, Capital Gold, Virgin and TalkSport.
The latest GfK results, out this week, suggest TalkSport's audience is three times as big as Rajar's diary system shows and LBC is twice as big - making it likely these would be the big winners if GfK comes out on top in the study.
TfL marketing director Chris Townsend said: "I believe the talk stations will benefit generally."
Morag Blazey, PHD's managing director, said the agency's massive research project was about more than MacKenzie's battle with Rajar. "I'm sure if GfK comes out of this as the most effective way of planning campaign, Kelvin MacKenzie will be thrilled - but that's not the purpose," she said.
Outgoing Rajar managing director Jane O'Hara said the industry body would not allow itself to be bullied. "If this is meant to put pressure on Rajar, we're not changing our stance just because someone has decided to do a test," she said.
Without doubt, however, the call for change is gaining ever more momentum, with the Media Research Group - a forum for media owners - this week surveying its members on the Rajar versus GfK issue.