A report released by Human Rights Watch also accused the British government of continuing to deport failed asylum seekers back to the country despite mounting evidence that they face the risk of being raped and tortured on their return, nearly four years after the end of civil war.
"This report is a challenge to existing UK government policy. The UK continues to deport Tamils to Sri Lanka a further charter flight is scheduled for this week despite evidence that some Tamils with alleged links to the Tamil Tigers have been tortured by the Sri Lankan authorities, following their removal from the UK," said David Mepham, the UK director of the international rights body.
"The UK should urgently revise its guidelines for assessing Tamil asylum claims, and not press ahead with removing at-risk Tamils," he added.
The Sri Lankan military has denied there had been any cases of rape or sexual harassments as described in the report.
'We Will Teach You a Lesson: Sexual Violence against Tamils by Sri Lankan Forces' runs into 141 pages and has been compiled over six years.
It provides detailed accounts of 75 cases of alleged rape and sexual abuse that occurred from 2006 to 2012 in both official and secret detention centres throughout Sri Lanka.
In the cases documented by Human Rights Watch, men and women reported being raped on multiple days, often by several people, with the army, police, and pro-government paramilitary groups frequently participating.
The United Nations Human Rights Council is set to examine whether the Sri Lankan government adequately followed up on it commitments in a March 2012 resolution to provide justice and accountability for wartime abuses.
The council is expected to pass a resolution calling on Sri Lanka to do more to work towards reconciliation and India is likely to support the motion.
Human Rights Watch has now called for the council to direct the UN High Commissioner for Human Rights to conduct an independent international investigation.
"The government's response to allegations of sexual violence by its security forces have been dismissive, deeming them as 'fake' or 'pro-LTTE propaganda'.
"It's not clear who in the government knew about these horrific crimes. But the government's failure to take action against these ongoing abuses is further evidence of the need
for an international investigation," Brad Adams, Asia director at HRW, said in a statement.
"The Sri Lankan security forces have committed untold numbers of rapes of Tamil men and women in custody. These are not just wartime atrocities but continue to the present, putting every Tamil man and woman arrested for suspected LTTE involvement at serious risk," he added.
HRW said most of the Tamils it interviewed for its report spoke with its investigators outside of Sri Lanka and researchers corroborated their stories with medical and legal records.
"Because Human Rights Watch was unable to openly conduct research in Sri Lanka or interview people still in custody, these cases likely represent only a tiny fraction of custodial rape in political cases.
"Many of the cases followed a pattern of an individual being abducted from home by unidentified men, taken to a detention centre, and abusively interrogated about alleged LTTE activities," the report said.
Sri Lanka has always denied allegations that it targeted civilians during the war and resisted calls for an independent inquiry, establishing its own Lessons Learned and Reconciliation Commission.