Fonseka will obtain the UNP membership at the party headquarters in Sirikotha on Thursday, the UNP said.
In February Fonseka was given the vacant UNP National List seat in Parliament.
The decision was taken when the UNP Working Committee met at the time to decide on the National List seat left vacant following the demise of M.K.D.S. Gunawardena.
Fonseka’s Democratic Party and the UNP had then reached an agreement to work together.
The former Army Commander contested the last Parliamentary election but failed to secure enough votes to enter Parliament.
He has been accused by human rights groups of being involved in war crimes during the final stages of the war. (Colombo Gazette)