NEW DELHI (Reuters) - Nitish Kumar, the chief minister of Bihar and once seen as a potential challenger to Prime Minister Narendra Modi, quit on Wednesday in a move that could shake up national politics.

The abrupt resignation followed a falling out with Kumar's coalition partner, Lalu Prasad Yadav, and torpedoed a state alliance that dealt Modi's nationalist party a rare regional defeat in an election less than two years ago.

"The situation is such that it is impossible to work any more," Kumar said into a scrum of TV cameras after tendering his resignation to the regional governor in the state capital, Patna.

The row centred around the role of Yadav's son as a regional minister, but its political ramifications could be far broader.

Kumar's grand alliance was seen by some leaders of India's political opposition as a possible template to challenge Modi at the 2019 national vote.

It was not immediately clear whether Kumar's resignation would trigger an election in the northern state or whether indeed he might team up with Modi's Bharatiya Janata Party (BJP) which is the main opposition party in the state legislature.