Two people have drowned and others have been victims of sexual assault while taking part in an international volunteering programme, the Government spending watchdog said.
Other volunteers have been detained by local police while taking part in the "high risk" programme, the National Audit Office (NAO) reported.
The International Citizen Service (ICS), launched by David Cameron, provides volunteering opportunities for 18 to 25-year-olds, while team leadership roles are available to people aged 23-35.
Phase one of the scheme, from April 2012 to August 2015, provided placements for 7,000 UK volunteers and the same amount from countries hosting projects.
The second phase, which began in September 2015 and runs until February 2019, has the target to provide placements for 10,000 volunteers from the UK, with a further 10,000 in-country.
The NAO report into the £173 million programme, which is contracted out to Voluntary Service Overseas (VSO), highlighted a number of concerns.
The report said the Department for International Development regarded ICS as a "high-risk programme due to the security and safety issues" involved.
Volunteers live and work in countries where they may be exposed to petty and violent crime, political instability, endemic diseases and natural disasters.
"Since phase two of the programme started, 3.75% of participants have been involved in incidents, including 0.24% who were involved in critical incidents and 0.07% in crisis incidents," the report said.
"ICS safeguarding incidents have included death by drowning of two volunteers, sexual assaults, and the detention of volunteers by local police."
The VSO has taken action against members of the consortium of organisations providing volunteering opportunities over safety concerns, the report said.
"Volunteers have experienced a variety of incidents in 2015 and 2016. VSO served notice under the provisions of its contract with one consortium member and paused the contract of another following security and safeguarding concerns," the report said.
A review of the programme in March 2017 concluded that VSO and the consortium agencies had made progress in developing a range of safety, security and safeguarding policies.
The NAO report concluded that ICS "has had some successes" - it was over-subscribed and meeting two out of three objectives.
But it had missed diversity targets and had "not consistently delivered development impact".
A DfID spokesperson said: "The NAO rightly highlights that the International Citizen Service programme provides a life changing opportunity for young people to help the world's poorest, fly the flag for Global Britain overseas and build their own skills, confidence and job prospects.
"The safety and wellbeing of all ICS volunteers is our first priority and this report acknowledges the actions we have taken.
"There are constant assessments and review of ICS programmes and we will not hesitate to stop working with partners that cannot meet our security standards."
Copyright (c) Press Association Ltd. 2017, All Rights Reserved. Picture (c) International Citizen Service.