Six Lok Sabha MPs from Shiv Sena (UBT) joined the Eknath Shinde-led faction in Mumbai.
The defecting MPs were absent from a crucial party meeting held five days prior in New Delhi.
Deputy CM Eknath Shinde described the development as the second phase of party expansion and the completion of Operation Tiger.
The shift impacts the opposition's parliamentary strength, following the 2024 general elections where Shiv Sena (UBT) won nine seats.