The announcement was made by Defence Minister A K Antony, who headed a four member observers team of AICC, which oversaw the exercise.
At a meeting convened to elicit the views of newly elected legislators, Siddaramaiah moved a resolution requesting that a decision on Chief Minister's post be left to Congress President Sonia Gandhi.
The central leadership, however, told the legislators to express their choice in writing, in a secret ballot.
Thereafter, Antony announced in the meeting that Siddaramaiah, who was Leader of Opposition in the Legislative Assembly, was the CLP leader.