Leading advertising executives have issued RTÉ with a stark warning to overhaul 2FM following the death of Gerry Ryan or face a massive loss in advertising revenue.

Ryan's 9am to noon slot is currently being filled by Lucy Kennedy and Colm Hayes, who got off to a shaky start last week. But advertising chiefs are warning that radio bosses must find an established personality to keep advertisers on board. RTÉ said it will make a decision on a permanent placement for Ryan within four weeks.

"Advertisers will want to make sure that every penny spent, especially nowadays, is being spent well," said Tony Frawley, managing director of Impact Media. "If they think that they are advertising on an ineffective programme they will go else­where, and that is how the business works. If media companies get a sense that listenership has gone down on 2FM, they just will not spend the money."

Industry sources also say the likelihood of RTÉ enticing well-known presenters from other stations is not high.

"There is a lot of talk about Ray D'Arcy and Ian Dempsey moving to 2FM but that is highly unlikely. They have established themselves with their own listeners and they would not be in the market for the impossible task of trying to replace Gerry Ryan. It is not going to happen. Only a fool would try and face the scrutiny which would come with that."

The combination of Lucy Kennedy and Colm Hayes has also been described as a mistake by one figure in the broadcasting industry.

"RTÉ have a big mess on their hands, and it was a big mistake putting Lucy and Colm in there. They have created two problems now. Not only have they put in a show which is not working and is ineffective, but they have destroyed another show at the same time, The Colm and Jim Jim Show."

Conor Boffin of First Advertising said advertisers are now watching carefully for any sign of a drop in audience numbers.

"The audience numbers will be very important. Big brands need big numbers. This will be important and another of the most important things for advertisers is the presenter. Either a presenter has it or they don't, and clearly Gerry did.

"Advertisers will be keeping a keen eye now on the audience age as well. 2FM seem to have made a decision to grow old with their audience. There is not really any one particular answer to the problem RTÉ has on its hands, but it needs a whole new strategy ? and our clients will look for this."

Tommy MacDonnell, direct­or of Ogilvy, said that other shows on the station will now be impacted. "Not only will the nine-to-noon slot be affected financially, but this will have a knock-on effect on the shows on either side too. Media buyers are watching closely right now to see how the numbers hold up."

Meanwhile, radio listeners gave their reaction online last week to The Lucy and Colm Show. One wrote: "It sounds like a bad version of the Spin morning show... not a worthy replacement for Gerry Ryan. But that's all Lucy. She needs to be sat down and sternly taught the basics by somebody, fast."

Another said: "The pairing of Colm and Lucy is absolutely terrible. Don't think they will last long - ad revenue will go down swiftly."