Singh passed the strictures at a meeting of the Modi-headed BJP Election Campaign Committee, state unit presidents and organisation general secretaries here.
Modi was present in the meeting.
The BJP Chief's comments have come against the backdrop of a resolution passed by the party's Bihar unit yesterday that Modi be declared the Prime Ministerial candidate by the Central Parliamentary Board without delay.
Other state units were specifically asked by the BJP president not to pass such resolutions.
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Party sources claimed Singh said in the BJP conclave that a decision in this regard will be taken by him after consulting all senior leaders.
The standard official line of the BJP has been that the decision on whether to declare a PM candidate, or whether it should be done before the Lok Sabha elections, as well as who should be the candidate, will be taken by the Central Parliamentary Board at the appropriate time.
BJP Parliamentary Party Chief L K Advani and Leader of the Opposition in Lok Sabha Sushma Swaraj, among others, are said to be against declaring Modi's name.