Dharmadasa's resignation as chairman of Sri Lanka Cricket's interim committee comes in the wake of a government decision to appoint Ranatunga.
Ranatunga, currently a ruling party legislator, led Sri Lanka to their lone World Cup triumph in 1996.
He quit international cricket in 2000 after playing 93 Tests and 269 one-day internationals.
He is expected to be appointed next week, sources close to President Mahinda Rajapakse's office have confirmed.
The interim committee will then be reconstituted, with the sports minister nominating members to the new panel.
Dharmadasa, a prominent local businessman, took over as chairman in 2005 after the then government decided to suspend the democratically elected cricket administration headed by Mohan de Silva.
.