A report by the Consumer Expert Group (CEG), a body set up to advise the government on the digital TV and radio switchovers, also warned the radio industry that it must not "bully" consumers into taking new digital technology.
The CEG called on the government to set up an independent body to provide the public with vital information on why they should voluntarily adopt digital radio services.
It added: "The target date for a digital switchover should be revised upwards as 2015 is realistically far too early for the necessary preparations to be put in place for consumers."
The report advised the government to run a full cost-benefit analysis of digital radio from a consumer perspective "as a matter of urgency", covering a range of "barriers" such as coverage issues and in-car vehicle conversions.
The CEG believes that no date should be set for the digital switchover until 30% of all radio listening is via analogue, compared to the current target of 50%.
"Setting a date, or a firm commitment to a date, would have had the effect of scaring consumers to switch. Clearly this would not be compatible with government policy to support a switchover when enough listeners voluntarily adopt digital radio," said the CEG.
"Given the slow speed of takeup so far, it is impractical to expect the remaining 50% to convert to digital radio in a two-year lead period. We are concerned that vulnerable listeners will be subjected to a marketing strategy to 'bully' them into adopting digital radio in the two years between an announcement and a switchover. This would risk considerable consumer resentment."
The CEG also questioned how effective switchover body Digital Radio UK has proved as a key source of "impartial information" for consumers.
It said that some of the body's marketing campaigns, including a recent Radio Amnesty scheme, have created unnecessary confusion among listeners.
According to RAJAR data, digital radio accounted for 24.6% of all listening in the second quarter of this year, against 67% via analogue AM or FM.
Owen Watters, sales and marketing director at radio manufacturer Roberts Radio, broadly welcomed the report's findings on the switchover timescale.
"While we unconditionally support digital as the platform for the future, we were the first manufacturer to raise concerns over the realistic timescales for the switchover," he said.
"[Culture minister] Ed Vaizey's statement in July was a key step forward in acknowledging this and we were in full agreement with his view that government will only press ahead with switchover, 'when the weight of public opinion is behind it'."
He added: "There is still a very healthy analogue demand in the UK and it is these consumers who will dictate when the switchover happens. It is also important to acknowledge that there is still also a lot of work to do before the UK is ready for the switchover, one major area of this being the need for a workable solution for the UK's 30 million motor cars.
"We are very excited about the future of DAB here in the UK and we are working hard with the Radio industry to get ready for the switchover, however as we have always expressed, it’s imperative that we remain realistic about the timescales, there's still a lot of work to do."