Pillai, who has been critical of Chief Minister Oommen Chandy's stewardship of the UDF over a host of issues, sent his resignation to the Chief Minister through a party functionary on Sunday, KC(B) sources said.
A long-time player in Kerala politics, Pillai courted fresh controversy last week with television channels airing the video of his telephonic talk with a leading hotelier in which he had purportedly asked the latter to go ahead with his allegations against Finance Minister and Kerala Congress (M) supremo K M Mani.
A minor constituent in the UDF, KC(B)'s lone legislator K B Ganesh Kumar was forced to resign from the ministry a year ago facing allegations of domestic discord.
After the case against his son was settled, Pillai had been pressing for re-instatement of his son as minister, a demand that has been glossed over by Chandy.