The UK government is refusing to release a report that it secretly commissioned into Bangladeshi prisons as concern grows ahead of a court appearance on Tuesday of an elderly British journalist being held in a notorious Dhaka jail.

Shafik Rehman, 81, will face a supreme court hearing over allegations of sedition. His family claim that the Foreign Office has effectively abandoned him and fears that, if charged and convicted, he could be sentenced to death. Even though no charges have been brought, Rehman has been detained for four months, during which his health has deteriorated. A prominent figure in Bangladesh, Rehman is a former BBC journalist and talkshow host and is the third pro-opposition editor to be detained in the country since 2013.

The commercial arm of the UK’s Ministry of Justice – Just Solutions International (JSI) – completed a consultation on Bangladesh’s prisons last year. However, the findings of the report have never been made public, despite concerns over the treatment of elderly prisoners. Freedom of information requests have been rejected by the MoJ on the basis of “protecting national security”, alongside diplomatic reasons.

Critics claim the UK government is effectively protecting Bangladesh by refusing to release potentially damning information about the conditions within its prisons.

JSI was forced to close earlier this year after winning a contract to train prison staff in Saudi Arabia. Set up by former UK justice secretary Chris Grayling, JSI had contracts with numerous governments with questionable human rights records, including Nigeria, Pakistan, Turkey and Libya.

Maya Foa, director of the death penalty team of legal charity Reprieve, which is representing Rehman, said: “By covering up these files, the UK government is helping Bangladesh whitewash its abuse and mistreatment of prisoners like Shafik.

“This 81-year-old British journalist spent a month in solitary confinement lying on the floor of a Bangladeshi prison cell as his health collapsed.He has now spent over 100 days in detention without charge and could face a death sentence, just for doing his job. The Foreign Office needs to urgently step up its assistance for imprisoned journalists like Shafik and support his release.”

Shumit Rehman, the 57-year-old son of the former journalist, said that he was afraid his father would never be free again. “I’m terrified that my dad’s health will fail,” he said. “He has a stent in his artery and had to be rushed to hospital once already. He’s missed important medical appointments in London. The UK government has information about conditions in Bangladeshi prisons that it is keeping secret. I want to know if ministers think these jails are safe for a frail old man like my dad.

“Instead of covering up poor conditions in Bangladesh’s jails, the Foreign Office should call for my father’s immediate release.”

A Foreign Office spokesman said it “continues to provide consular assistance” in the case.