The 26 lawmakers, led by Martin Amaewhule, are loyal to the immediate Governor of the state and Minister of the Federal Capital Territory (FCT), Nyesom Wike.
Wike and Fubara have been at loggerheads over the political power control in the oil-rich state.
A Port Harcourt High Court had issued an order prohibiting the lawmakers from presenting themselves as members of the Rivers State House of Assembly following their defection.