Pravin Mahajan, who was sentenced to life imprisonment for killing brother-BJP leader Pramod Mahajan, lapsed into coma following a suspected brain haemorrhage, his family said on Saturday.
Pravin is critical, his brother Prakash Mahajan said.
Pravin was rushed to Jupiter Hospital, Thane, late Friday night following complaints of high blood pressure.
He had been released by a court on parole from the Nasik Central Jail two weeks ago on health grounds. The parole ended Saturday.
Pravin was convicted by a Mumbai court in December 2007 for killing his elder brother Pramod at his home in Worli on April 22, 2006.