"We will happily welcome Mani," Achuthanananda told reporters here.
As Leader of the Opposition in the assembly, he would not oppose any such move, the Marxist leader said.
On whether Communist Marxist Party (CMP) led by M V Raghavan and JSS led by K R Gowri would also be welcomed into CPI(M)-led Left Democratic Front, Achutanandan said let those parties make their stand clear on the issue.
Mani's remarks that "coalition relationships are not permanent" had triggered speculations that he was unhappy with the UDF.
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Speaking to reporters in Thiruvananthapuram, Mani said he had made the comment only on ideological basis on queries about relationships of parties in a coalition.
Thanking Achuthanandan for his offer, Mani said such a thought (of leaving the UDF) had not crossed his mind.
Home Minister Thiruvanchoor Radhakrishnan said in Kozhikode that Achutanandan's offer was only based on speculations as Kerala Congress (M) was still with the UDF.
Kerala Congress (M) leader and Water Resources Minister P J Joseph dismissed as hypothetical a question about leaving the UDF.