More than one in eight men do not volunteer to work with children because they are worried people will think they are a paedophile, a survey suggests.
The charities say young boys need positive role models
Childrens' charities NCH and Chance UK also say almost one in five men do not come forward because they would have to undertake a criminal records check.
More volunteers will have to be checked because of a new child protection law being introduced next year.
The survey found 69% of men do not give up their time to help youngsters.
The charities organised the Volunteer Survey 2007, which interviewed 1,019 adults.
Work commitments were given as a reason by 59% of those who did not volunteer, and 68% said they did not have enough time.
In all, 13% of the men questioned said they would not choose to volunteer to work with children due to fear of being perceived as a paedophile.
Both the NCH and Chance UK are calling for men to come forward and mentor boys aged five to 11.
The NCH's chief executive, Clare Tickell, said: "Many children, especially boys, are desperately in need of a male mentor, which is why we urgently need men to come forward despite any fears they may have about public perception.
"We work hard to ensure volunteers are checked by the police, trained and monitored, which we hope encourages men to come forward and helps assuage the public's concern."
Jo Hobbs, development manager at Chance UK, said: "Male volunteers are more difficult to recruit, yet positive male role models can make a huge difference to the lives of challenging children and young people.
"We urge anyone out there interested to get in touch to find out more."