Sinha said his new assignment as an advisor in the Chief Minister's Office (CMO) had led him to quit the panel.
"After taking up the new responsibility as advisor to the Chief Minister, I did not feel it right to continue as the (AAP) Lokpal. So, I told Arvind (Kejriwal), that I won't be able to discharge the duties as Lokpal," Sinha told PTI.
AAP spokesperson Deepak Bajpai confirmed that Sinha had stepped down in order to take up the new assignment.
With Sinha moving to the CMO, the three-member AAP Lokpal panel is left with only one member now. The panel is thus currently headed by SP Verma, an educationist.
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Earlier, former IPS officer N Dilip Kumar had quit the post citing "personal reasons" and other engagements, but ruled out any differences with the party leadership.
AAP is yet to fill the vacancies that have arisen in the three-member panel.
After cracking the whip on dissident leaders Prashant Bhushan and Yogendra Yadav, AAP had reconstituted its Lokpal panel while shunting out former Lokpal, L Ramdas.
Ramdas fell out with the party leadership after he was purportedly sent an SMS asking him to not attend the controversial National Council meeting that saw Bhushan and Yadav being sacked from the national executive of the party.