Dozens of people calling Samaritans were put at risk by rogue volunteers-including so-called sexual predators.
In the past four years, the charity had to investigate 46 alleged violations of protection rules.
It is understood that it reported the most serious situation to the police.
A source said: “Many Samaritans have been taking advantage of callers to meet with them outside of shifts.
“Compared with the excellent work done, these numbers are small, but they should not be concealed.”
Conservative MP Nigel Mills said last night: “This is very worrying.
“People go to Samaritans when they are most vulnerable, and it’s shocking to hear that someone is being used.”
In a confidential memo to volunteers, senior management stated that 13 of the 46 incidents were serious.
They say the most worrying is the failure to safely enforce policies and procedures, sexual misconduct, and interacting with callers when they are not on duty.
The memo added: “Every time a caller is in danger in some way.”
Charity More than 20,000 volunteers provide a 24-hour helpline for callers facing emotional stress or suicidal thoughts.
They handle 10,000 calls for help every day and are bound by strict confidentiality rules.
Charity, whose patron is Prince Charles, Insist that all security incidents are dealt with vigorously and reported to the charity committee.
It is introducing other measures to strengthen the process to volunteers.