He made the claim on Tuesday at a discussion in Dhaka, saying he had a copy of a letter written by a Pakistan army officer on his iPad.
"Ziaur Rahman worked as a Pakistani agent. The letter written by Pakistan army officer Brigadier Beg is the proof," the ruling party's publicity and publications secretary said.
Zia went on to become Bangladesh's first military dictator and later founded the BNP, which is currently headed by his wife Khaleda Zia.
He rehabilitated the Jamaat-e-Islami, which spearheaded anti-Bangladesh efforts during the war, in politics after assuming power. His regime also freed about 11,000 alleged anti-independence supporters.
Two war crimes convicts were even made ministers in Khaleda’s 2001-06 cabinet.
His elder son Tarique Rahman, also the party's senior vice chairman, has recently made headlines by making controversial claims about the Liberation War and the nation's founding father Bangabandhu Sheikh Mujibur Rahman.
But it is not the first time Zia has been called a spy of Pakistan.
In 2010, then the state minister Qamrul Islam made a similar claim. The BNP dubbed the comment "baseless and violation of political etiquette".