The I-T department has recieved complaints of benami properties against Kumar, in which it's alleged that BSP chief's brother has a share in benami properties of real estate players.
The authorities have sent notices to some builders and are probing an alleged nexus between them and Kumar.
It comes as a huge shock for Mayawati, who is eyeing her comback to power in UP after five years.
Earlier today, Mayawati claimed that the prospective alliance between the SP and the Congress UP will only be sealed after the approval of the BJP.
Addressing a press conference in the state capital, Mayawati said: "There is widespread speculation that these two parties would form an alliance to fight the forthcoming state polls together but this cannot happen unless the BJP okays it."
The former Uttar Pradesh Chief Minister also alleged that these three parties were conniving together to keep her party out of power.
"The BJP is on the wrong foot ever since Prime Minister Narendra Modi announced demonetisation and it can go to any length to prevent a surging BSP from coming to power," she said.