Bhagat alleged that the producers of the film have not given appropriate credit to him.
Bhagat, who is the highest selling Indian writer in English, said the '3 Idiots' producers assured him the movie had an original script and was only 2-5 per cent based on Five Point Someone, the novel that catapulted the author to fame.
However, when the film released, Bhagat alleged that it borrowed heavily from the book.
"What's with the court threat. I'd happily go to court. They'd lose so badly. Because there, truth talks, not stature," he wrote on micro-blogging site Twitter.
"I wanted to see the final script – it was never shown to me. I wanted to see the film before release – it was not shown to me (even though trials had been done for people).
+"What's more, the makers had called me to their office and pressured me several times to withdraw my ‘Based on a novel by' credit, which was by contract. They told me they'd replace it with something like ‘initiated by' – a credit that doesn't exist anywhere in the world," the author said in his blog.
Earlier, Aamir Khan had said: "I think he (Bhagat) is trying to get publicity to sell more copies of ‘Five Point Someone'. In fact, I told Vinod (producer of the film, Vidhu Vinod Chopra) that he should take him to court as he is maligning both Vinod and Raju (director Raj Kumar Hirani)."
"It's Abhijat Joshi, who has been working on the script of the film for the last three years. We borrowed the idea from Bhagat's novel, but frame-to-frame it has no relation with the book. Ask him to pick up a single scene that is similar to the book and then we'll talk. It's Joshi's hard work," Aamir told reporters in Kolkata.
A sizable section of the Indian media has backed the author. Journalist Vir Sanghvi has supported Bhagat in this controversy.
"I am on the side of Chetan Bhagat in the '3 Idiots controversy. Authors deserve prominent credit if their books are sources," Sanghvi tweeted Friday.