An appeal against the controversial acquittal of a British-born militant convicted of murdering American journalist Daniel Pearl opened at a Pakistani court on Tuesday.

A Karachi court sparked outrage earlier this year when it overturned the 2002 murder conviction of Ahmed Omar Saeed Sheikh, and acquitted three other men connected to the case.

Pearl's parents and prosecutors lodged an appeal at Pakistan's Supreme Court in May, putting the release of the four men on hold.

"The case has finally opened, it will be decided whether they should be convicted or acquitted. The case is heading to a final verdict," Faisal Siddiqui, the lawyer representing Pearl's parents, told AFP.

The appeal, which has been frequently postponed in recent months, will hear opening arguments in the capital Islamabad on Wednesday.

Sheikh had been on death row for Pearl's murder but was acquitted in April by the Sindh High Court which instead sentenced him to seven years for kidnapping -- paving the way for him to walk free after already serving 18 years.

Three co-defendants who were serving life sentences in connection to the case were acquitted.

Pearl was South Asia bureau chief for The Wall Street Journal when he was abducted in Karachi in January 2002 while researching a story about Islamist militants.

A graphic video showing his decapitation was delivered to the US consulate nearly a month later.

Pearl's killing stirred international condemnation of Pakistan's military government just as it was remaking its image after years of backing the Taliban in neighbouring Afghanistan.