Seven Aam Aadmi Party (AAP) Rajya Sabha MPs, forming over two-thirds of its strength, have defected to the Bharatiya Janata Party.
AAP has termed the move illegal and unconstitutional and will seek their disqualification.
The party cited the Tenth Schedule of the Indian Constitution, which does not recognise splits and allows only mergers with two-thirds support.
The decision on disqualification lies with the Chairman of the Rajya Sabha (Vice-President of India).
Anti-defection law (added by 52nd Amendment, 1985) aims to prevent political defections; members can be disqualified for voluntarily giving up party membership or defying party whip.