The Association of Private Children's Homes claimed all of its homes were owed fees by the council. It said that a considerable number had substantial sums owing and the worst case is in excess of pounds 100,000.
Heather Underwood, the association's secretary, said that it would sue in the High Court for non-payment of debt, other costs and interest if large sums remain unpaid.
Ms Underwood said about 150 children, aged between 10 and 16, all deemed to be at risk for reasons of abuse or lack of care, were housed from Lambeth. The association also houses children from other areas, mainly in London.
A spokesman for Lambeth said that the problem was caused by the private care organisations submitting their bills too slowly. 'They have left it for two, three or four months,' he said. 'There is a backlog and we are working on it. We are now monitoring payment orders more closely to prevent this happening again.'
The association said that a number of homes have been placed in a desperate position due to Lambeth's inefficiency.
Ms Underwood has asked Virginia Bottomley, Secretary of State for Health, to take up the matter. She said Lambeth's incompetence was 'affecting the whole spectrum of children in care'.
'One should draw the conclusion that Lambeth social services department is putting the majority of the children in their care at risk by putting their placements with carers in jeopardy,' she said.