Union minister Ravi Shankar Prasad accused the Rashtriya Janata Dal chief of breaking law by "shamelessly" hobnobbing with a man convicted of murder and accused in over 30 serious cases, saying the development was text book case of constitutional impropriety.
"Nitish Kumarji, a crime has been committed. Are you going to launch criminal proceedings against your ally Lalu Prasad?" he asked at a press conference.
Bihar BJP President Nityanand Rai demanded Kumar's resignation, saying the report has exposed the criminal- government nexus in Bihar.
Prasad used the occasion to take a swipe at the opposition parties over their attempts to forge unity against the ruling BJP.
"I want to ask the secular brigade which is working overtime to form an alternative. I will ask Sonia and Rahul Gandhi, Sharad Yadav and Sitaram Yechury if they will compromise with such heinous crimes to expand their secular umbrella," he said.
A news channel on Saturday played purported telephonic conversation between Lalu and Shahabuddin, a former union minister and senior office bearer of the RJD, in which the Siwan leader is heard complaining against the local SP and asking him to take action. The RJD chief is then heard asking an aide to connect him to the SP, Prasad alleged.
Lalu Prasad wields big influence in the Bihar government and Shahabuddin continues to "run" administration in Siwan and neighbouring areas, he said.
Prasad said Shahabuddin was then in Siwan jail. The Supreme Court later ordered his transfer to Delhi jail following a petition from a Bihar-based parents whose three sons were killed, allegedly at Shahabuddin's behest.
The BJP leader noted that the jailed gangster is accused of murdering some Left leaders as well as he targeted its leadership over their attempts to ally with the RJD.
Rai said the local administration officials in Bihar should be dismissed and Kumar should resign.
"It has now become clear that Nitish is in alliance with people who are used to breaking the law. Action must be taken against Lalu Prasad," he said.
The incident has showed that Kumar can ally with anybody to be in power and it was time for him to go, he added.
Prasad claimed that country's polity was moving away from corruption and crime but the development showed leaders like Lalu Prasad continue to practice the same old politics.
Face to face with the dreaded Mohammad Shahabuddin
Siwan strongman Shahabuddin walks out of jail to a hero's welcome
Shahabuddin's 'selfie' inside Siwan jail triggers controversy
The cost of worshipping Shahabuddin
'I will fight Shahabuddin till my last breath'