“The criminals responsible for the barbaric and murderous attack on Sarabjit Singh must be brought to justice,” Indian Prime Minister Manmohan Singh said on his official Twitter page.

India complained that its diplomats were denied access to the prisoner as he fought for his life, and the premier said it was “particularly regrettable” that Pakistan had not responded to appeals “to take a humanitarian view of this case”.

The prime minister added that New Delhi would make the necessary arrangements to bring his body home for funeral rites, after earlier negotiations to treat the jailed spy in India or a third country failed.

Pakistan insists regular consular access was granted to Singh and said doctors did everything possible to save him before his death from cardiac arrest.

“The prisoner, who had been in a comatose state and on a ventilator for the last few days, was being provided the best treatment available and the medical staff at Jinnah Hospital had been working round the clock... to save his life,” the foreign ministry said.

The government provided “all assistance” to Singh’s family and the Indian authorities, and will facilitate “the early completion of all formalities” and hand over his body “at the earliest possible” time, it added in a statement.

Singh’s lawyer Owais Sheikh said his body had been moved to the hospital mortuary. The doctor said arrangements were under way for an autopsy.

Two prisoners were taken into custody immediately after the attack and have now been charged with his murder, police official Tariq Mehmood said.

The motive was unclear, but Pakistani police say an initial investigation pointed to an exchange of “hot words” with Singh.

Sheikh said his client had received threats following the execution of a Kashmiri separatist in India. Mohammed Afzal Guru was hanged in New Delhi in February for his part in a deadly attack on the Indian parliament in 2001.

Pakistani analysts doubted the death would have a significant impact on tense relations between New Delhi and Islamabad, nuclear-armed neighbours.

“It may increase tension for a while as the hawks in India could put pressure on the Indian Congress [ruling party] to criticise Pakistan for lack of security,” retired lieutenant general Talat Masood said.

“I think Pakistan will hand over the body to his family, it will be helpful in calming the situation and will surely be viewed as a good gesture.”

Singh was convicted for his alleged involvement in a string of bomb attacks in Pakistan’s Punjab province that killed 14 people in 1990. His mercy petitions were rejected by the courts and former president Pervez Musharraf.

His family insisted he was a farmer who became a victim of mistaken identity after inadvertently straying across the border while drunk.

The Human Rights Commission of Pakistan condemned the attack as a “dastardly act” and called on the government to punish those responsible.

“The authorities have obviously failed to do their elementary duty” of keeping him safe, the commission said in a statement.

Singh’s wife, two daughters and a sister have returned to India after travelling to Lahore on Tuesday, according to the .

Pakistan last year released an Indian man who completed his sentence of three decades in a Pakistani jail on espionage charges.