"I will launch a regional party Dec 10. The same day I will quit BJP and also resign as assembly member," he told reporters here after 'Audhya pooja' at his office, which was formerly the BJP office.
The former chief minister said preparations for the launch of the new party will formally start Wednesday as it is 'Vijayadashami' day. He said the the new party would be launched in the presence of "lakhs of supporters at a public meeting in Haveri, about 350 km north of Bangalore.
"My decision to leave the BJP and launch a new party is firm. I appeal to my supporters not to give any credence to reports that I will change my decision and stay in the BJP," he said.
Yeddyurappa said he had not decided on when his supporters in the Shettar ministry and legislators backing him should quit the BJP.
He claims the support of around 70 of the party's 120 legislators in the 225-member assembly. Nearly half of the ministers in the 34-member Shettar cabinet are Yeddyurappa supporters.
The former chief minister is clearly wary of being held responsible for bringing down the Shettar ministry as both he and Shettar hail from the Lingayat community, which makes up for 17 percent of the state's 65 million population and is generally believed to support the BJP.
With Yeddyurappa fixing the date of his party's launch, focus will now be on how many ministers and party legislators will attend the Dec 10 show of strength and whether BJP leadership will act against them.
The BJP leadership has been wavering on acting against Yeddyurappa though he has been ridiculing it for months.
With assembly election due in May next year, the party may choose to ignore the act of ministers and legislators attending the launch of a new party instead of risking the fall of Shettar ministry.
That could entail President's rule till the assembly polls, which the BJP would like to avoid as it has often accused Governor H.R. Bhardwaj of acting as a "Congress agent".
The 69-year-old Yeddyurappa, largely credited for leading the BJP to power for the first time in south India, says the BJP leadership has "betrayed" him.
He resigned as chief minister in July last year over mining bribery charges and faces a dozen corruption and illegal land deals cases. He was succeeded by D.V. Sadananda Gowda in August 2011. However, he has been asserting that BJP president Nitin Gadkari did not keep the promise to make him state BJP president within 24 hours of quitting as chief minister.
Shettar succeeded Sadananda Gowda in July this year.
Yeddyurappa has also been holding BJP leader L.K. Advani responsible for the mess. Yeddyurappa and Shettar were known rivals. Shettar was not even made a minister when Yeddyurappa formed the ministry in May 2008.
He had opposed Shettar when there were moves to make him the chief minister after he (Yeddyurappa) quit. But in less than a year, Yeddyurappa fell out with his successor Gowda, brought him down and backed Shettar's claim to chief ministership.