Patient care has not been improved as a result of doctors' surgeries ceasing to provide out-of-hours services, an overwhelming majority of GPs believe.
Not having to provide out-of-hours cover has eased the workload
A study by public spending watchdog Audit Scotland found just 11% of GPs felt the changes had helped patients.
It warned that the way the service is provided risks becoming unsustainable and must be changed.
GPs have been able to opt out of providing late-night and weekend care since 2004, with 95% choosing to do so.
Responsibility for providing out-of-hours care is passed from practices who choose to opt out to the local NHS board.
Providing this out-of-hours care cost NHS boards an estimated £67.93m in 2006-07.
However, Audit Scotland found that only one in nine doctors thought patient care was better under the new arrangements.
In addition, 52% of the GPs believed that patient access to and the availability of out-of-hours services had not improved.
However, Audit Scotland found that more than 80% of the patients who had used out-of-hours healthcare were satisfied with the service they received.
Also, 88% of GPs were relieved that they no longer had 24-hour responsibility for their patients.
The report warned: "There is a significant risk that current models of out-of-hours service delivery are unsustainable.
"The way in which services are delivered must change if out-of-hours services are to be sustained."
Dr Dean Marshall, chairman of the BMA's Scottish general practitioners committee, said ministers had not appreciated the "personal and financial burden borne by GPs" under the previous system.
He added: "The cost of delivering services out-of-hours is now being borne by NHS boards who are struggling to maintain services at existing levels."
However, he said that the change had benefited GPs, adding: "Before the new contract was introduced, morale was at an all-time low, GPs were planning to retire early and recruitment was becoming impossible.
"Now, however, GPs have more control over their workload, work-life balance and morale has improved."
Public Health Minister Shona Robison said: "In order to maintain and improve the delivery of out-of-hours services, the Scottish government will work with boards to extend the role of nurses, paramedics and others in developing the service."
Tory health spokeswoman Mary Scanlon said she was "deeply alarmed, but sadly not surprised" by Audit Scotland's report.
She added: "There is now an impending crisis as fewer and fewer GPs opt to provide their services to NHS 24, and the inherent danger that far less qualified medical staff will take over their responsibilities."
Leaders of the trade union Unison welcomed the report but said resources must be made available if other health professionals were increasingly relied upon to provide out-of-hours care.