The five - who are deponents to the Mukherjee Commission probing the mystery surrounding Bose's whereabouts after the Taihoku air crash in 1945 - also wrote to Sharmila Tagore, Censor Board of Film Certification chairperson, and the secretaries of the union home and information and broadcasting ministries this week.
They are opposed to the fact that the film, to be released worldwide next Friday, shows Netaji - as Bose was fondly called by his admirers - as married to Austrian woman Emily Shenkel.
"We have corroborative proof that Netaji's marriage to Shenkel is a concocted story," Jayanta Chowdhury, one of the researchers, told.
According to him, although Netaji's family members claim he got married to Shenkel in 1937, his application for a visa to
"Despite our appeals to refrain from controversial issues, Benegal went ahead with the portion depicting Netaji as a married person. The film was telecast on German television," he said.
Two years ago, the same group - comprising Chowdhury, Rudrajyoti Bhattacharjee, Madhusudan Pal, Surajit Dasgupta and Nandadulal Chakraborty - had sent a legal notice to Benegal asking him to refrain from showing that Netaji died in an air crash.
They now threaten to move court if action is not taken on the romantic scenes showing Bose.
"If the romantic scenes showing a married Netaji are not removed despite our requests to the Censor Board and the ministries, we would move the Calcutta High Court to stop the film's release," threatened Bhattacharjee, a lawyer.
Added Chowdhury: "Bose's family members are out to prove that Netaji was married to Shenkel.
"Late Sisir Bose (Netaji's nephew) said he married in 1942. In 1994, the Bose family released 162 love letters of Netaji (written to Emily). Where did those letters come from after so many years?
"Netaji's 'daughter' Anita criticised the Mukherjee Commission, but herself did not agree for a DNA test to prove her claim. We don't say that Netaji was above marriage. But it is also not true that Netaji could marry at that time," he insisted.
The film, made with a budget of Rs.220 million, traces the life of Bose from 1941 to 1945 and the exploits of the Indian National Army founded by him in the backdrop of World War II.
Producers Sahara One Motion Pictures are planning to organise a mega premiere in Kolkata's sprawling Eastern Command Ground Thursday to make an entry into the Guinness Book of World Records on the basis of audience numbers.
Former Trinamool Congress parliamentarian Krishna Bose, wife of Sisir Bose, complimented the film for having brought out the leader's life very well.
"It is a moving film and it brings out the person that he was. The representation is dramatic, but authentic," she told.
"When Shyam Benegal approached us, we knew the subject was in safe hands. We had opened our family archive to him for research. I think the director has done a good job," Krishna Bose noted.