Lawyers for the five are to refer the case to the United Nations Human Rights Commission as a breach of the International Convention on Civil and Political Rights. The Yemeni government is a signatory to the treaty.
The lawyers say the case is "one of the worst examples" of British subjects abroad being ill-treated.
Dr Christopher McIlroy, an accredited Home Office pathologist at the University of Sheffield, met the prisoners in Yemen at the request of the defence. He was not able to carry out a full medical examination but was able to have a close look at their injuries, which showed they had suffered "serious physical ill-treatment".
Dr McIlroy said his conclusions contradicted the official version of the state of the prisoners' health and an independent medical examination was urgently needed. The prisoners said they had endured severe beatings, electrocutions, food and sleep deprivation and sexual abuse.
Yesterday, the Yemeni government was accused of blocking a visit to the prisoners by the prominent civil rights lawyer Stephen Jakobi, despite his visa application being backed by the Foreign Office.
Mr Jakobi, of the Fair Trials Abroad Trust, who has joined the defence team, said the Yeminis' action ended the possibility of him seeing the detainees. "The applications were made last week and the Foreign Office also sent a letter to the Yemeni government. We spoke to the consul general of Yemen in London and they were fully aware of the time schedule," he said.
"[Yesterday] we were told by the Yemeni government that they knew nothing about my application and we must begin the whole process again. This is a deliberate attempt to sabotage the defence lawyers by the use of delaying tactics.
"What we have is one of the worst cases of torture, including sexual abuse, of British prisoners abroad and there is obviously serious concern for their wellbeing."
The Yemeni government alleges that the five Britons - Mohsin Ghalian, 18, Samad Ahmed, 21, Ghulam Hussein, 25, Malik Nassar Hahra, 26, and Shahid Butt, 33 - and a French national, James Luovres, are linked to the Islamic fundamentalist Sharia group and to Abu Hassan, who is accused of kidnapping 16 tourists.Reuse content