"The Speaker's ruling is unfair, blatantly partisan and in direct conflict with law and statutes," party spokesman Anand Sharma told PTI.
He said that citing past conventions and Speakers' direction, the present Speaker cannot invoke the same to overrule the law of Parliament and statutes.
"Any convention and Speakers' directive cannot subordinate the law," he said.
Sharma said the Speaker's decision "squarely goes" to prove that the present regime is going to "undermine" every institution including Parliament.
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"I have gone by rules and tradition," she told PTI on the decision.
Congress has been maintaining that there is no law which make it binding for the largest opposition party to have a particular number of seats to claim the LoP status in Lok Sabha.