The government's new independent assessment service (IAS) for checking employees who have been off work through ill heath for more than four weeks is likely to be delayed.
It is also understood that it is unlikely to be a nationwide service when it is unveiled.
The service was originally proposed by the sickness absence review in 2011 and later confirmed by government with the intention of being launched in October 2014.
However, one of the members of the steering group for the project confirmed that there were still some key questions to answer before the IAS could be rolled out.
EEF chief medical adviser Professor Sayeed Khan said the process was taking longer than expected and there were still many important questions to answer.
"It is taking time. It is very sensitive this whole thing and it isn't as clear as we thought it might be," he said.
"One of the problems is going to be if you have someone going to your occupational health (OH) service and getting some advice and then being sent by their GP to the IAS and having different advice. What do you do? This is why it's been really difficult, so don't hold your breath about when it's going to come out.
"And do you really think we're going to have a national service from day one? If you were going to see how to fail abysmally; launch something nationally from day one," he added.
However, speaking at a Jelf employment seminar, Prof Khan added that the inclusion of tax relief for back to work interventions was a major step for the government.
"Tax relief is a big step - it's a really big thing. It's such a tiny little bit that they've done, but it's such a big step for government and it's the start," he said.
Presenting earlier, Jelf head of benefits strategy Steve Herbert suggested that referral to the IAS could become mandatory for anyone off for more than four weeks.
He also raised other questions facing the IAS.
"When will the IAS begin? In the original document it said October last year or even in April - yet right now we don't have much detail at all, so the chance of it coming out in spring next year is pretty close to zero because this is a huge job," he said.
"Even 12 months might look like a leap of faith," Herbert added.
"And this tax incentive - will it only apply to treatments recommended to the IAS? Because that's how the consultation document was written by HMRC," he concluded.
