Latest media reports say that the Samajwadi Party leader took this step on his doctor's advice.
Speaking to CNN-IBN on his decision, Amar Singh said that health concerns such as blood pressure and kidney, and also his desire to spend more time with his family led him to take this step.
Further, he has said that he will remain a party member.
The sudden move has raised questions if the resignation came due to some sort of political pressure.
Mulayam Singh Yadav is yet to accept the resignation.
A formal press release on the issue is scheduled at 4 pm in Delhi.