"She (Kangana) is on board. She has liked the script. It's role of a lifetime. She is at the top now... She is doing good work. I think she was best for the film," Mehta told PTI.
To get into the skin of her character, Kangana will be undergoing training in horse-riding and sword-fighting.
Mehta is returning to the pre-independence era following his Aamir Khan-starrer biopic on Mangal Pandey in 2005.
"The process of casting is on and we will be making an announcement soon," Mehta said.
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The director, known for films like "Mirch Masala", "Maya Memsaab", "Rang Rasiya", said the biopic will go on floors next year.
Talking about the project, Mehta said, "Rani Laxmibai is one of the most fascinating women I have come across in terms of what she has done in her life. I enjoy making biopics.
"It is a great platform to say stories of those people who have done great work. It has been a challenge to write this biopic but these are good challenges. I am very excited to start working on it," Mehta said.