Plaid Cymru wants a comprehensive re-evaluation programme
Millions of pounds of public money aimed at helping the most deprived areas of Wales are being sent back unused, it has emerged.
Research by BBC Wales reveals that over £6m of Communities First funds was returned over the last two years.
Plaid Cymru is calling for an urgent review.
The assembly government said some money which is not used or reallocated in any year does remain unspent but said these were normally relatively small amounts.
Over the last five years since it was set up the Communities First programme has received £136m helping deprived areas.
Information released to BBC Wales' The Politics Show under the Freedom of Information Act shows that while cash is getting to the communities, some 13% of it has not been able to be used.
Project co-coordinators said the reason was that many staff members leave their jobs and while trying to replace them, the funding was left unused.
One of the original architects of the Communities First programme, Professor Dave Adamson, said he believed a shortage of trained workers and relatively high turnover was to blame.
"The programme was launched in effectively 142 partnerships across Wales in one big bang if you like to use that term - and inevitably there weren't the trained and qualified workers on the ground," he said.
"Many people moved into it from kind of related areas in local government and health et cetera and I think quickly realised the jobs weren't for them and left very very soon after," said Prof Adamson.
"So the initial high turnover was very much a product of the shortage of trained and skilled development workers. Unfortunately that shortage still prevails."
Wales has some of the most deprived areas in Western Europe
Leanne Wood, Plaid's social justice spokeswoman, is calling for an urgent review.
A Welsh Assembly Government spokesperson said: "The Communities First Budget is allocated to all the 142 eligible areas in Wales.
"Each area receives an offer letter that includes funding towards core staff who facilitate the programme in each area
The spokesperson said under spends may result from a wide variety of circumstances such as staff being on sick, maternity and special leave, or normal short term vacancies which occur when staff leave posts before new appointments can be made.
"In such circumstances there can also be slippage in the training budgets allocated to the staff.
The spokesperson added: "The under spends from one year cannot be rolled forward to following years but the partnerships in the respective areas are given the opportunity to spend these monies on other eligible activities within the year wherever possible and considerable additional work has been funded in this way since the start of the programme."