The charity has contacted the DWP expressing concerns about the practice, which it says has the effect of treating volunteer centres as free subcontractors.
Volunteering England, a membership organisation for the centres, claims the practice contravenes the Merlin Standard, the code of conduct for organisations involved in the programme’s supply chain.
The code says prime contractors must use "fair and transparent" procurement processes and should ensure funding arrangements are "fair, proportionate and do not cause undue financial risk for supply chain partners".
Volunteering England has been contacted by centres in 14 prime contract areas saying they have received referrals without payment.
In a briefing paper on its website, the charity names the prime contractors it says are involved.
In a letter seen by Third Sector, Chris Grayling, the Secretary of State for Work and Pensions, wrote back to Volunteering England saying: "Providers and their subcontractors should not be approaching voluntary organisations if they have not entered into, or are intending to enter into, an agreement with them and they are outside the supply chain."
Grayling said the department could not interfere in the business relationship between prime contractors and subcontractors, but it expected all prime contractors to acquire Merlin accreditation during the first year of the programme.
"DWP will contact Work Programme providers to ensure they are aware of our position on this," he wrote.
Dan Sumners, senior policy and information officer at Volunteering England, said centres did not always know who had referred clients, so it was impossible to know how many had been sent by prime contractors.
But he said centres had reported an increase in workloads since the programme had begun. Some centres are formal, paid subcontractors.
Sumners said long-term unemployed people referred by prime contractors often had complex needs that required more intense help at a time when many local authorities had reduced funding to the centres.