The post became vacant after the demise of the Speaker G Karthikeyan on March 7.
"The decision to field Sakthan has been taken. No decision was made on who should be fielded as the deputy Speaker," Thankachan told reporters after the UDF meeting at the official residence of chief minister Oommen Chandy.
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Meanwhile, Kerala Congress (M) has raised its demand for the post of deputy Speaker, but it is learnt the Congress is in no mood to accept it.
Earlier, there had been rumors the Congress party was considering several names for the post of Speaker, but most of its leaders and legislators, however, had backed Sakthan's candidature.
Sakthan, 63, is a member of the Nadar community, which has a sizeable population in the Thiruvananthapuram district.
A former minister, also a four-time legislator from the capital district, Sakthan has been the deputy to Karthikeyan since 2011.
Sakthan's election to the post is likely, as the 140-member state Assembly has UDF commanding 74 seats while the Left Front Opposition has 65 seats, and one seat is vacant following Karthikeyan's death. Election to the post of Speaker will be held on March 12. The state government has issued a notification on the election.