Nicholls has been the voice of Formula E since the championship began back in 2014 and has provided commentary for BBC Radio 5 Live on Formula 1 since 2016.
Ahead of the Jakarta E-Prix double-header this weekend, Formula E confirmed that Nicholls would step down from his role with immediate effect but cited no reason for the split.
It has subsequently emerged that Nicholls was sacked after an external investigation by an employment specialist following "complaints of inappropriate behaviour".
A spokesperson for the electric racing championship said: "Formula E can confirm that an investigation was carried out in response to complaints of inappropriate behaviour received about Jack Nicholls. Following this investigation, Jack Nicholls's contract to provide race commentary was terminated."
It's understood formal complaints were made in March by three people, with Nicholls providing commentary remotely from London instead of being out on location with the championship while the investigation was conducted.
Nicholls said: "Although disappointed with the decision, I respect it and accept why it was taken.
"I want to take full responsibility for what I did and apologise unreservedly for a couple of isolated incidents that has made those concerned feel uncomfortable.
"I never meant any harm and I am committed to making amends and to be more mindful of my behaviour in future."
Ben Edwards, the ex-F1 commentator for Channel 4, will step in to provide commentary this weekend in Jakarta.
"I am very excited to get back behind the mic with Formula E as the on-track action this season is a commentator's dream," said Edwards.
"The championship is more competitive than ever and I get to bring that to life for viewers around the world. I'm delighted to be joining the team and calling the action in Jakarta."
Nicholls' departure comes after long-time presenter Vernon Kay confirmed he was leaving his role to focus on his BBC Radio 2 show, with Radzi Chinyanganya stepping into the position this weekend.
Saunders Carmichael-Brown will take on Chinyanganya's pitlane reporter duties.
