The deputy director of the death penalty team at the Reprieve organisation, which is supporting Ibrahim Halawa, has said that there is clear evidence that Ibrahim has been tortured while in prison in Egypt.
Mr Halawa, who is from Dublin, was arrested by the Egyptian army as he took refuge in a Cairo mosque while Muslim Brotherhood protesters staged a "day of refuge" outside in August 2013.
This week he is spending his 1,000th day in prison.
Speaking on RTÉ's News At One, Harriet McCulloch said the Reprieve team has visited him and have seen the marks of torture on his body.
She said the Irish consular offices have also recorded serious marks and bruising on his body.
She said the United Nations Working Group on Arbitrary Detention and the UN expert on torture have concluded that they have serious concerns about Mr Halawa being tortured.
Ms McCulloch said they have also expressed grave concerns about him being held in excessive pre-trial detention.
She said Reprieve is concerned that the 20-year-old student is facing a possible death sentence even though he was a child at the time of the alleged offence.
She also said she has grave concerns that he does not have proper access to a lawyer.
Ms McCulloch said the circumstances surrounding his detention are not acceptable.
Meanwhile, Amnesty International has met with the Egyptian Ambassador to Ireland, Soha Gendi, to discuss the case and to reiterate its call for Mr Halawa's release.