Nicola Sturgeon has written to the embattled foreign secretary, Dominic Raab, urging him to intervene in the case of a Scottish Sikh man who has been imprisoned in India for nearly four years awaiting trial, and is facing the death penalty after a confession allegedly extracted under torture.

In Sturgeon’s first formal intervention on the case, seen exclusively by the Guardian, the first minister expresses the Scottish government’s “deep concerns” about Jagtar Singh Johal’s detention without trial – as well as his allegations of torture and mistreatment by Indian authorities while in custody.

The letter adds further pressure on the UK government after nearly 140 parliamentarians, including the former Brexit secretary David Davis, the former international development secretary Hilary Benn and the SNP leader at Westminster, Ian Blackford, wrote to Raab in February asking him to seek Jagtar’s release.

Jagtar is being supported by the legal NGO Reprieve, whose deputy director Harriet McCulloch described his situation as “as clear a case of arbitrary detention as we’ve come across”. She said that, despite numerous court appearances, his trial has been repeatedly delayed at the request of the prosecution, with basic information denied to his defence counsel.

McCulloch said that Jagtar, a Sikh human rights activist from Dumbarton, was violently arrested in Punjab, northern India, in October 2017, after which he was tortured with electricity into confessing his involvement in an alleged terrorist conspiracy.

It has been alleged he provided GBP3,000 to a Sikh who was planning to kill members of the extremist nationalist group Rashtriya Swayamsevak Sangh (RSS), a charge he denies.

In the letter, Sturgeon urges Raab to personally meet Jagtar’s family after her own meeting with his brother Gurpreet Singh Johal this month.

“Gurpreet expressed his disappointment with the support his brother and their family have received from the FCDO [Foreign, Commonwealth and Development Office]. He also questioned why the British prime minister did not raise Jagtar’s case with the Indian prime minister in April this year.”

Boris Johnson has endeavoured to cement economic ties with India as part of the UK government’s wider foreign policy strategy by holding online talks with its prime minister, Narenda Modi, in May and during the G7 summit in Cornwall in June.

Gurpreet Singh Johal told the Guardian: “I am very grateful for the Scottish government’s support, but my brother really needs the Westminster government to seek his release and bring him home.

“Our family is devastated that almost four years after he was abducted and tortured, the Foreign Office has still not fulfilled its promise to take ‘extreme action’ on his case. I hope the foreign secretary will meet with me and do everything in his power to save Jaggi’s life.”