A judge in an Islamabad anti-terror court last week granted bail to Zakiur Rehman Lakhvi, accused of the siege in India's commercial capital that left 166 people dead and was blamed on the banned militant group Lashkar-e-Taiba (LeT).
The bail decision triggered a furious response from New Delhi and prosecutors swiftly announced they would appeal against it.
"We will file a petition in Islamabad High Court next week," prosecutor Mohammad Azhar Chaudhry told AFP.
Chaudhry had originally planned to file the appeal on Monday, but had to delay it as he had not received a written copy of the bail ruling.
Lakhvi remains in custody in the high-security Adiala jail in Rawalpindi after the authorities ordered his detention under public order laws following the bail decision.
Indian Prime Minister Narendra Modi told lawmakers last week that the bail order came "as a shock to all those who believe in humanity world over".
New Delhi has long said there is evidence that "official agencies" in Pakistan were involved in plotting the attack.
Islamabad denies the charge but LeT's charitable arm Jamaat-ud-Dawa, seen as a front for the militant group, operates openly in the country.
Seven Pakistani suspects have been charged with planning and financing the attacks but the failure to advance their trials has been a major obstacle to better ties between Pakistan and India.
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raider : I am appalled at your ignorance. Samjotha perpetrators are already in prison.
why govt is going to challenge if he does not posses any singlet evidence, Pakistan should also put pressure on India to arrest perpetrators of samjohtha express carnage
Let's see for how long Pakistan can hoodwink the world to its own peril. Waiting for another tragedy perhaps before taking a call in this matter?