"Sushil Modi and Nitish Kumar should explain as to how without their permission how such a decision was taken," Singh said.
Singh is probably the first senior Bharatiya Janata Party leader, who has raised this issue in public so far. He said everyone has failed to understand the reason behind the letter.
"In Bihar government is run by JD-U and BJP, RSS is our main organisation and Sushil Modi is Deputy Chief Minister. He should be asked as to how it happened.
"What was the motive behind such move and why the need arised to gather information on state functionaries of the RSS," Singh said.
Singh also taunted Nitish Kumar, saying how it was possible that such a big decision was taken without his permission. It is a matter of a probe.
"What happened was objectionable, party workers and leaders as well as people are angry about it," he added.
Singh said order to gather information of the RSS and its affiliates was not proper.
Ruling JD-U leader Dilip Singh has strongly reacted against Singh, saying he should meet Prime Minister Narendra Modi and BJP president Amit Shah in this connection.
Bihar Police has already hinted to take action against some police officers in connection with a letter as a damage control after BJP leaders questioned it.
After hue and cry made by the BJP leaders over the letter, Chief Minister Nitish Kumar has directed the state DGP, Home secretary, ADG (CID) to act against police officers responsible for issuing such letter without informing the department.
So far, police headquarters has maintained that letter was issued without the knowledge of concern top police officers.
The letter was issued by Superintendent of Police (Special Branch) on May 28, two days before the swearing-in of Narendra Modi as the Prime Minister for the second time.
( With inputs from IANS )