Siddiqui has last week sent his resignation to the Congress vice-president Rahul Gandhi alleging that party had failed to come upto the expectations of the common man.
Siddiqui, who was in SP had joined Congress during 2012 assembly polls and had lost Lucknow central seat to SP candidate Ravidas Mehrotra.
SP spokesman Rajendra Chowdhury, who appeared before media with Siddiqui said that his joining the party would strengthen it.
Asked about the suspension of IAS officer Durga Shakti Nagpal, Chowdhury said that the matter was in knowledge of Chief Minister Akhilesh Yadav and he would do the needful.