By Zubair Ahmed
BBC News, Mumbai
A senior leader of India's Hindu nationalist Shiv Sena party has quit citing differences with his cousin, who is the party's executive president.
The Shiv Sena was split in July when Narain Rane was expelled
Raj Thackeray said he would form a new party next month.
In July, another key figure, Narain Rane, was expelled from the party, leading to an exodus of supporters.
Analysts say Raj Thackeray's move may further weaken the far-right party, particularly as this latest crisis involves a Thackeray family member.
Shiv Sena is led by Bal Thackeray, uncle of Raj Thackeray.
It is traditionally strong in the western Indian state of Maharashtra.
Raj Thackeray raised the banner of revolt a few weeks ago when he resigned from all the party posts, again over differences with his cousin, Udhav Thackeray.
His open defiance suggested that the Shiv Sena chief Bal Thackeray's vice-like grip over the party seems to have been loosened.
But the party's fortunes had already suffered a big blow when Mr Rane was expelled due to anti-party activity in July.
Mr Rane joined the Congress party along with the party's thousands of ordinary workers.
He even won a by-election recently defeating a Shiv Sena candidate.
But some analysts say it is too early to write the Shiv Sena's obituary.
Party supporters are disappointed with their leaders for allegedly abandoning the party's right-wing Hindu philosophy, but they believe the leaders will make amends soon and go back to their original agenda.
There was no comment from the Shiv Sena.
The alliance of the Congress party and the National Congress party beat an alliance of the Shiv Sena and the Bharatiya Janata Party in the elections in Maharasthra state in 2004.