Boris Johnson has for the first time said that the Indian government is arbitrarily detaining Jagtar Singh Johal, the British Sikh activist held in an Indian jail for four and a half years.

In a letter to Keir Starmer seen by the Guardian, the prime minister says Singh has been arbitrarily detained without formal charges being laid against him. He was arrested in 2017 over his alleged role in killings by the Khalistan Liberation Force (KLF), a banned terrorist organisation.

His brother Gurpreet Singh Johal, who had a meeting with the Labour leader and the shadow foreign secretary, David Lammy, on Thursday, hailed the admission as a breakthrough but asked why it had taken so long.

A UN panel on arbitrary detention last month also acknowledged that Jagtar Singh Johal has been arbitrarily detained, without any prosecution started.

Johnson defended the UK government’s approach to the detention and added that ministers had consistently raised concerns about Johal’s treatment and right to a fair trial.

He said the issue had been raised with the Indian government almost 100 times since Johal’s detention in November 2017. In the letter, he said the case was raised by the foreign secretary, Liz Truss, with the Indian external affairs minister, Dr Subrahmanyam Jaishankar, during visits to India in October 2021 and March 2022. Johnson added he had personally raised the case directly with the Indian prime minister, Narendra Modi, on his visit.

Gurpreet Singh Johal said: “I was grateful for Keir’s intervention and for him to meet earlier today. He very quickly understood that time is of the essence and if the UK government do not act then a British national could be given the death penalty based on false allegations and manufactured charges.

“This is a breakthrough moment. I will never forget that it took the UK government almost five years to acknowledge that my brother is arbitrarily detained, or that they only did so after a nudge from the UN and the leader of the opposition, but at least they got there in the end. The next step is to demand his release and bring him home.”

Jagtar Singh Johal, from Dumbarton, was arrested after travelling to the Punjab for his wedding. Several trial dates have since been postponed or cancelled and he is yet to stand trial.

The degree of high-level cross-party interest in the case is now likely to put pressure on the wider British relationship with India. The UK has been pursuing a relationship based on closer trade, and trying to woo a reluctant India towards a defence alliance more hostile to China.

But with the UK government staging a two-day conference on religious freedom next week, some ministers are likely to want to raise the way in which Muslims and Sikhs are being treated by Hindu extremists in India.