Amitabh Bachchan has threatened legal action over a YouTube video that apparently shows the Bollywood actor championing Narendra Modi as India's next prime minister.

Bachchan described the online video as "fake" on Wednesday and expressed outrage on his Twitter and Facebook accounts.

The 70-year-old actor said the video featured footage from a 2007 'Lead India' newspaper campaign but added visuals to suggest he was promoting the Bharatiya Janata Party (BJP) leader.

"I wish to state that this is an illegal act, inviting strong action, one that I am appalled by, and one that has no concurrence from me at all," Bachchan said on Twitter. "Infuriated and angered."

The YouTube video has since been taken down.

Bachchan has appeared in a series of advertisements promoting tourism in the state of Gujarat, of which Modi is the chief minister, but has never openly supported the Hindu nationalist leader.