UK intelligence agencies are accused of tipping off Indian authorities about Jagtar Singh Johal, a British national, before his abduction and alleged torture by Punjab police, the media reported.
Johal, from Dumbarton, was in India in 2017 when his family say he was forced into an unmarked car, the BBC reported.
He says he was then tortured over days, including with electrocution. He has remained in detention since then.
Successive British Prime Ministers have raised his case but India’s government denies he was tortured or mistreated, says the BBC.
In May, Johal was formally charged with conspiracy to commit murder and being a member of a terrorist gang.
He will be presented with a full list of the charges against him next month and faces a possible death penalty.
Now the human rights group Reprieve has shown the BBC documentation that it says is compelling evidence that his arrest followed a tip-off from British intelligence.
The UK government says it will not comment on an ongoing legal case.
Reprieve says it has matched several details relating to his case to a specific claim of mistreatment documented in a report by the watchdog that oversees the intelligence agencies.
“In the course of an investigation”, says the Investigatory Powers Commissioner’s Office (IPCO) report, “MI5 passed intelligence to a liaison partner via the Secret Intelligence Service (MI6).
“The subject of the intelligence was arrested by the liaison partner in their country. The individual told the British Consular Official that he had been tortured.”
Johal is not named in the report, but Reprieve’s investigators are adamant the facts match his case due to the dates concerned, the lobbying by British Prime Ministers and supporting evidence detailed in the Indian press, the BBC reported.