The merger will mark a complete break of RSP with the Left block though its leaders claim the party would strive for a broader alliance of secular and democratic forces, including Congress, as a "credible" alternative to NDA at the Centre.
Two MLAs of RSP, including its state secretary A A Azeez, would be sitting in UDF benches in the assembly when its monsoon session begins on Monday.
In return for alloting Kollam seat to RSP, Congress in the state had extracted a promise that if got elected Premachandran would remain part of UPA at the Centre.
RSP(B) faction, led by state Labour Minister Shibu Baby John, has for long been with UDF and the two factions would merge to become a single party on June 10.
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Asked about the party's approach towards the Left block at the national level, Azeez said "we stand for strengthening the unity of the Left and secular forces with Congress playing a key role."
He said RSP all-India secretary T J Chandrachoodan, West Bengal unit secretary Kshiti Goswamy and national council member Sukumar Ghosh would attend the "re-unification" meet.