Agha also condemned keeping Omar's death a secret for two years calling it a "historical mistake",
Agha released a statement that announced his resignation on Monday less than a week after the group announced the appointment of a new Taliban leader, Mullah Akhtar Mansoor after it emerged that Omar died in 2013.
What does Mullah Omar's death mean for the Taliban and Afghanistan?
"It was needed that the leader of Islamic Emirate [the Taliban] had to be appointed in the presence of the courageous Mujahideen in their strongholds inside the country [Afghanistan]," Agha said in his statement.
"The appointment of every leader that has taken place outside the country has brought very bad repercussions."
News of Agha's resignation has sparked speculation over the unity of the group and its future decisions.
"One of many reasons Tayyeb Agha’s stance is important: it lends weight to rumours of [Mullah] Yaqoob and [Mullah Abdul] Mannan not being happy with Mansoor’s selection," Mujib Mashaal, an Afghan journalist, said on Twitter.
Born Syed Tayyab Agha Popolzai in Afghanistan's Kandahar province, Agha had earned the respect of the Taliban leadership because of his closeness to Omar as one of the former leader's most trusted aides.
Agha was also regarded as the Taliban’s leading negotiator in ongoing peace talks.
He was the Taliban's most articulate representative, and can speak English and Arabic. He is in his late thirties.
According to sources in Afghanistan, Omar trusted him with highly sensitive information and Agha acted as his English-language interpreter and envoy before the US invasion in 2001.
"The appointment of every leader that has taken place outside the country has brought very bad repercussions."
News of Agha's resignation has sparked speculation over the unity of the group and its future decisions.
"One of many reasons Tayyeb Agha’s stance is important: it lends weight to rumours of [Mullah] Yaqoob and [Mullah Abdul] Mannan not being happy with Mansoor’s selection," Mujib Mashaal, an Afghan journalist, said on Twitter.
Born Syed Tayyab Agha Popolzai in Afghanistan's Kandahar province, Agha had earned the respect of the Taliban leadership because of his closeness to Omar as one of the former leader's most trusted aides.
Agha was also regarded as the Taliban’s leading negotiator in ongoing peace talks.
He was the Taliban's most articulate representative, and can speak English and Arabic. He is in his late thirties.
According to sources in Afghanistan, Omar trusted him with highly sensitive information and Agha acted as his English-language interpreter and envoy before the US invasion in 2001.
