
One-time Modi mentor LK Advani and three other veterans of his right-leaning Bharatiya Janata Party (BJP) issued a statement that appeared to accuse the premier of failing to seek consensus, without naming him directly.
Waning popularity: Modi suffers defeat in crucial Bihar election
"The principal reason for the latest defeat is the way the party has been emasculated in the last year," the joint letter released to the media said.
"A thorough review must be done of the reasons for the defeat as well as the way the party is being forced to kow-tow... and how its consensual character has been destroyed," the letter added.
The octogenarian figures have largely been sidelined by Modi since his landslide victory in May 2014, but have not spoken out in such a public forum until now.
It came as the premier takes stock of a humiliating defeat last Sunday in one of India's most populous states, Bihar.
Modi had addressed dozens of rallies, attracting thousands of people, in the key east Indian state and promised voters billions of dollars for social and economic projects, but to no avail.
Advani, a former deputy prime minister, was one of the nationalist BJP's founding members, and is widely credited for leading the party's expansion in its formative years.
Giant projection reading 'Modi Not Welcome' displayed on British Parliament
He is also recognised as one of the few leaders who groomed Modi for power, until they reportedly fell out over competing political ambitions.
Observers said that the revolt posed no direct challenge to Modi's authority unless it inspired other, younger leaders to speak out.
"Logical to join BJP revolt against Modi if 2014-19 is your last realistic shot at power. Daft if you have a longer political time horizon," Sadanand Dhume, resident fellow at Washington-based American Enterprise Institute, told AFP.
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