An elderly British journalist investigating corruption in Bangladesh has been arrested by a police squad that received UK aid funding despite allegations of torture.

Shafik Rehman, 81, was seized in April on accusations of plotting to kidnap and kill the prime minister's son. He has been held without charge for three months, and his family fears he faces the death penalty if convicted.

The former BBC contributor, who has dual British and Bangladeshi nationality, is a prominent opposition figure. He is also credited with introducing Valentine's Day as a holiday to the mainly Muslim country.

Shafik Rehman, 81, (middle) was seized in April on accusations of plotting to kidnap and kill the prime minister's son. He has been held without charge for three months

Britain is Bangladesh's biggest aid donor. This year it is handing over £154 million, despite growing repression and a free-speech crackdown that has seen three leading opposition journalists arrested since 2013.

'This is the problem with aid all over the world – there is no accountability,' said Mr Rehman's son Shumit, who runs a tuition company in North London. 'But ultimately Britain is responsible if it is paying these people.'

His father, who trained as an accountant and edits a popular magazine, was taken by police posing as a TV crew then interrogated for ten days, his family say.

Mr Rehman was forced to sleep on the floor in solitary confinement for a further 15 days, despite worsening health problems. He suffers from diabetes and has a stent in his artery.

His cell in a notorious prison is next to another prominent journalist who has been held without charge for three years. Mr Rehman, who has three grandchildren in Britain, has told his family he fears the same fate.

He was arrested by Bangladesh's detective branch, which is accused by Human Rights Watch of being responsible for serious abuses 'including arbitrary arrests, torture, enforced disappearances and killings'. Yet in 2009, Britain gave £10 million towards a five-year United Nations scheme to enhance the unit's ability to deal 'professionally' with investigations.

The police training continued until seven months ago. British aid to Bangladesh includes £33.63 million for a 'safety and justice programme', despite a warning last year that such support 'could be used for political purposes and/or human rights violations'.

Britain is Bangladesh's biggest aid donor. This year it is handing over £154 million, despite growing repression and a free-speech crackdown that has seen three leading opposition journalists arrested since 2013

The watchdog said UK aid may be helping intelligence efforts 'used to monitor and suppress political opposition groups'.

Bangladesh has been sliding into authoritarian rule and was hit by a series of Islamist murders in recent months. Mr Rehman's family, who called the allegations 'farcical', believe his arrest was linked to an advisory role he took with the main opposition leader earlier this year.

Maya Foa, of anti-death penalty charity Reprieve, which has taken up the case, said: 'Britain must demand answers from Bangladesh on whether UK aid has contributed to the arrest of journalists like Shafik Rehman.'

The Department for International Development said aid to Bangladesh police stopped last year, adding: 'It is wrong to suggest DFID funding contributes to human rights violations. The UK Government is committed to protecting human rights and holding to account those responsible for the worst violations and abuses.'