The debt charity said 18% of its clients owed £3,530 on average to a friend or family member in 2010 after turning to them for extra support because they could not keep up debt repayments.
This figure represented more than one sixth of clients' total average unsecured debt of £22,476, according to the charity.
The average monthly repayment agreed by clients who borrowed from family or friends was £124 - making up one of the five or six debt repayments that the typical person contacting the charity was struggling to meet each month.
The charity started collecting this information in 2008 following anecdotal reports from its debt counsellors of an increase in this type of lending, possibly as a result of restricted credit conditions during the recession.
Delroy Corinaldi, director of external affairs at CCCS, said: "Many people turn to family members or friends for a loan when they are struggling to cope - especially where obtaining credit is difficult because of a poor credit history.
However, borrowing in this way can place an enormous strain on friendships and family relationships, and should be considered very carefully."