"Massive shortages" of education welfare officers could undermine efforts by schools in Wales to tackle truancy, it is claimed.
Truancy expert Prof Ken Reid, of Swansea Metropolitan University, said there was a particular shortage in support for primary schools.
Education welfare support is far behind levels of 25 years ago, he said.
Education Minister Leighton Andrews said he had put a new emphasis on behaviour and attendance.
Mr Andrews said in a speech earlier this month that Wales had had the worst school attendance levels in the UK for years.
Prof Reid, who has led research into behaviour and attendance for the assembly government, has previously raised concerns about truancy in Wales.
He is particularly concerned about the problem of truancy in primary schools, which he said was a relatively recent phenomenon.
He told BBC Wales' Week In Week Out programme: "Unfortunately there are massive shortages of education welfare officers throughout Wales.
"Some authorities have more than others.
"There is a particular shortage in support for primary schools, but generally speaking, the levels of education welfare support now available are a quarter of the levels they were around 25 years ago."
Mr Andrews said there was "excellent practice" in Wales, but admitted there was "shocking variation between and within schools and local authorities".
He has promised "zero tolerance" of truancy.
"Certainly I've put a new emphasis on behaviour and attendance issues," he said.
"These are clearly areas we need to get right.
"Indeed, if we could get the levels of attendance to that of England's, then on average, secondary school pupils would get an extra eight weeks training during the course of their school lifetime, so I think this is something we have to make good on."
Attendance is one of 20 key areas outlined by the minister in his plan to turn around Wales' school results.
A report by schools inspection body Estyn last month said standards in nearly a third of schools in Wales were not good enough.
In December, international assessments in reading, maths and science suggested Welsh children were lagging behind the rest of the UK and much of the world.
Speaking after the publication of the assessments by the Organisation for Economic Co-operation and Development (OECD), Mr Andrews said there was "systemic failure" in the school system and accused teachers of "complacency in the classrooms".
Week in Week Out is on BBC 1 Wales on Monday, 15 February, at 2235 GMT.