The Shiv Sena chief will be the first from the Thackeray family to be sworn in as the chief minister.
H D Deve Gowda said that Governor should have ascertained whether Fadnavis had the majority of MLAs to back him as chief minister.
The decision came as a surprise as deliberations among Congress, NCP and Shiv Sena on the formation of a non-BJP government in the state had seemingly reached the final stage.
The letter was signed by Eknath Shinde, Jayant Patil and Balasaheb Thorat, the legislature party leaders of the Shiv Sena, NCP and Congress, respectively.
The two-day Conference of Governors and Lieutenant Governors commenced on Saturday at Rashtrapati Bhavan with the inaugural address of President Ram Nath Kovind.
The three parties demanded an immediate floor test to avoid 'further horse-trading' and filed a petition in the SC seeking quashing of Maharashtra governor's decision to swear-in Fadnavis as CM.
Hard at work was the BJP national general secretary Bhupendra Yadav, who had been tasked by the top brass with completing the formalities of ushering in a government led by Devendra Fadnavis.
The Sena said the BJP, which is in power at the Centre, left the affected farmers in the state 'disappointed' with the 'meagre' relief.
CM Mamata Banerjee said that the central and state governments are elected by the people and the federal structure must work as per Constitution.
Congress leader Randeep Singh Surjewala said the President's Rule was 'a joke with democracy' and was an ill deed against the established traditions of the Constitution.
Ex-CM Prithviraj Chavan said that the governor should have invited his party to indicate 'willingness and ability' to form the government.
Aaditya Thackeray had on Monday said that governor was told that his party was willing to stake a claim but its two potential allies needed time to complete the formalities for extending support.
While expressing its inability to form government in Maharashtra, the BJP blamed Shiv Sena for insulting the mandate given by the people of the state.
Sensing that the numbers are not on its side, the BJP declined to form the government. It also blamed its ally Shiv Sena for insulting the people’s mandate.
The BJP won 105 seats in the October 21 elections, while the majority mark in the 288-member Assembly is 145.