Shiv Sena chief Bal Thackeray underwent an angiography on Tuesday morning and is likely to operated upon, a party insider said.
Thackeray, 83, was admitted to the Lilavati Hospital in Mumbai on Monday following breathing problems.
After the angiography and the medical panel's expert advice, Thackeray may undergo a surgery later Tuesday, a party insider said, requesting anonymity.
The Lilavati Hospital had set up a five-member panel to treat him - Jaleel Parker, who has been regularly treating him; JJ Dalal, Samuel Mathew, Ajit Menon and Prakash Jindani, who examined the senior leader to determine the course of treatment Monday.
Concerned about his advanced age and frail health, the medicos have discussed the treatment options available with Thackeray's family.
Son Udhav has cut short his trip to the US and is expected return to Mumbai later Tuesday to be with his father, who underwent a heart surgery a few years ago.
Thackeray's estranged nephew and Maharashtra Navnirman Sena chief Raj Thackeray, senior party leaders like Manohar Joshi, party MPs, legislators and senior leaders of the ally Bharatiya Janata Party (BJP) have been thronging the hospital to inquire after his health.
Thackeray had been admitted to the same hospital June 18 for treating similar respiratory problems, causing concern among family and party members. He was discharged after six days in hospital.
Earlier, in March this year, Thackeray had spent 11 days in Lilavati Hospital for similar health complaints.