Council homes in Gwynedd were transferred to Cartrefi Cymunedol Gwynedd Cyf (CCG) in April 2010 when the housing body started refurbishment work on 6,000 homes.
Ffrancon Williams, the chief executive of CCG, said one of the contractors being used was leaving the project due to a ‘number of factors’.
He told the BBC: “This is a labour-only contract, whereas contractors would usually tender for the labour and materials, so we have learned lots of lessons.
“As a result of this some tenants have suffered and we apologise for this,” he added.
Properties services group Apollo, the contractor which is leaving the project, said no-one was available to comment.