Giving a jolt to the LDF, RSP came out of the Front last Saturday after they were denied the Kollam Parliamentary seat by the CPM for the coming Lok Sabha polls.
UDF convenor P P Thankachen told reporters after a leadership meeting of the Front that RSP has been allotted the Kollam seat.
He also said RSP has also given an assurance to the Front that they would support the UDF and the UPA at the Centre after the polls.
The RSP, a long-time partner in LDF, broke off with it on March 8, protesting the "overbearing" approach of the CPM and the CPI in seat allocation.
The party also decided to put up its former MP N K Premachandran as its candidate in Kollam, its traditional stronghold.
It had also rejected the appeal of central leaders of the CPM and the CPI to reconsider its decision in the larger interests of Left unity.
Kollam was once a RSP stronghold and the party had sound trade union base in the area which had several traditional industries like cashew processing.
Though RSP's founder leader late N Sreekantan Nair had won from Kollam repeatedly, the CPM took over the constituency later and had won in 2004.
Premachandran, who was a minister in the previous LDF ministry led by V S Achuthanandan, had won twice from there.
In 2009, CPM candidate was defeated by N Peethambara Kurup of Congress.