Pakistan has asked India to explain why one of its nationals, who went to India to watch a cricket match in 2005, was arrested and later died in an Indian prison, a Foreign Office spokesman said on Wednesday.
Khalid Mahmood, 30, died on February 12 but his family in Pakistan was only informed of his death on March 4, according to family members, who said he had been imprisoned as a spy and was tortured to death.
A Foreign Office spokesman said Indian authorities had not informed Pakistan about Mahmood's arrest until he died in what the spokesman said were mysterious circumstances.
"The death of Mr Khalid Mahmood in custody should be condemned in the strongest terms," spokesman Mohammad Sadiq told a regular briefing.
"We approached ... Indian authorities to give us a detailed report of the circumstances, the reasons for which he was arrested, the circumstances of his death," he said.
Mahmood's body was sent back to Pakistan on Monday and one of his brothers told Reuters on Tuesday Indian authorities had not informed the family how his brother died but his body bore signs of torture.
The brother said Mahmood had lost his passport while in India to watch cricket and was later picked up by Indian police.
Old rivals India and Pakistan have improved their relations after nearly going to war on fourth time in 2002 but they are still deeply suspicious of each other.
On March 4, Pakistan released an Indian, Kashmir Singh, who spent 35 years on death row in Pakistan on spying charges but was released after President Pervez Musharraf accepted his mercy plea.
Sadiq said Singh had been released on humanitarian grounds and Pakistan had hoped the gesture would be reciprocated.
"It is very unfortunate that very soon we received a dead body of a Pakistani national from India," he said.
But Sadiq said the incident would not affect a peace process the neighbours launched in early 2004.
As part of the peace process, both sides began sending cricket teams to each other's country and thousands of fans have travelled back and fourth to watch the teams clash since then.