In parliament, parties united this morning to condemn Pakistan and Home Minister Rajnath Singh said, "Basic norms of law and justice were violated. I want to tell the house that the government will do whatever it takes to make sure Kulbhushan Jadhav gets justice."

Foreign Minister Sushma Swaraj slammed the death sentence as "an indefensible verdict" and warned Pakistan to "consider the consequences" of the case on its relations with India.

Pakistan's Defence Minister Khawaja Asif has claimed that "rules, regulations and laws of the land" have been followed in a three-month trial before the death sentence was announced. "We have done nothing that violates the law... We are not involved in any premeditated murder," he said, adding, "We will defend our country at all costs." He added that Mr Jadhav has the right to appeal against the sentence within 60 days.

Mr Jadhav, a former naval officer, was arrested by Pakistan in March last year in the turbulent province of Balochistan, which has seen a long-running conflict between Pakistani security forces and a militant separatist movement. Pakistan's military alleged that he had confessed to being tasked by India's intelligence service with planning, coordinating and organising espionage and sabotage activities in Balochistan "aiming to destabilize and wage war against Pakistan".

The death sentence was confirmed by Pakistan's powerful army chief General Qamar Javed Bajwa. India has outright rejected Pakistan's account of Mr Jadhav's activities; it summoned Pakistan's high commissioner Abdul Basit on April 10 in protest; officials pointed out that Indian diplomats in Pakistan were repeatedly refused access to Mr Jadhav.

India says that Mr Jadhav had been kidnapped in Iran and "his subsequent presence in Pakistan has never been explained credibly". The Home Minister said in parliament that Pakistan claimed that Mr Jadhav had been found with a valid Indian passport. "Jadhav ran a small business in Iran and travelled to Tehran in connection with this," he said.

"If Pakistan hangs Kulbhushan Jadhav, then it is murder. The government will be seen as weak if it cannot free him," said Congress leader Mallikarjun Kharge. "The government has influence, please use it. Will the government make every effort to save him? It is our joint responsibility to save him," said Hyderabad lawmaker Asaduddin Owaisi.

After the news of Mr Jadhav's sentence broke yesterday, India retaliated by refusing to release 12 Pakistani prisoners who were due to return home today after completing their jail term.

When Mr Jadhav was arrested, India said the claim that he was a spy was "baseless" and said a video of his alleged confession was not authentic. Pakistan says he was deployed by top Indian intel agency RAW or the Research and Analysis Wing.