Christian Aid is concerned that sexual attacks are going unreported in Colombia.
The UK Government was asked to raise the issue of sexual violence in Colombia at a United Nations Universal Periodic Review of the country's human rights situation this week.
Thomas Mortensen, country manager of Christian Aid in Colombia welcomed the recent G8 declaration on Preventing Sexual Violence in Conflict, which stated that there should be no safe haven for perpetrators of sexual violence in armed conflict.
However, he said the situation in Colombia remained "pressing".
Louise Winstanley of human rights groups ABColombia further warned: "Although peace talks are underway after five decades of conflict in Colombia, there is a vast under reporting of cases of sexual attacks that occur, and a failure to prosecute that amounts to impunity."
She wants to see greater governmental support for civil society organisations working with victims of sexual violence. These organisations provide legal advice, documentation of their cases and psychosocial support.
Christian Aid highlighted the case of a human rights worker, Elizabeth, who was raped while visiting a rape victim to record her testimony. The victim's husband had been killed by paramilitaries and she had been raped, resulting in pregnancy. Three hooded paramilitaries entered the home while Elizabeth was there and raped the victim again to prevent her talking. Elizabeth too was raped. She says she was later attacked again and survived two kidnap attempts.
"Imagine how difficult it will be if the first person a woman has to report to is a member of the military, and the person she is accusing is part of the same institution," said Winstanley.
"A recent legal change in Colombia put evidence-gathering for conflict-related sexual crime in the hands of the military.
"We want the UK to urge Colombia to modify the new law, so that the military are not responsible for the collection and evaluation of information, or we will face an increase in impunity."
Of the 183 cases highlighted by human rights lawyers in the last five years, just four have been brought to trial.
Human rights defenders are putting themselves at risk to help victims, with 69 killed last year, according to Alirio Uribe, a Colombian lawyer helping victims of violence in Colombia and who is involved in the UN's Universal Periodic Review.