The former senior officer is seeking to give evidence to a parliamentary inquiry about how MI5 officials witnessed detainees being tortured at Camp X-Ray and Camp Delta — two of the Guantanamo prisons — in December 2002.
Even though the former officer is trying to get official permission to give evidence, it is thought to be unprecedented for a former member of staff to defy the agency in this way.
Senior security sources have told The Sunday Times that the testimony will prove for the first time that MI5 was fully aware detainees at Guantanamo were systematically abused and tortured.
Details of torture were disclosed during meetings held at the London headquarters of Britain's MI5 in 2002 and the evidence is believed to include claims that British officials witnessed inmates being chained, hooded, waterboarded and subjected to mental abuse by CIA officials.
A report by the US Senate published in 2014 said the CIA used sexual threats, waterboarding and other harsh methods to interrogate terrorism suspects in the years after the Sept. 11, 2001, attacks on the United States.
A former British inmate at Guantanamo said last month that British security officers witnessed him being tortured by American soldiers in Afghanistan.
Separately, a former British inmate at Guantanamo said in 2014 he underwent psychological torture included execution threats and light deprivation.
The prison in Guantanamo, in Cuba, was opened in 2002 to house foreign terrorism suspects but has drawn international criticism from human rights activists and many foreign governments. US President Barack Obama is seeking to close it.