The Democratic Party, led by the former army commander, Sarath Fonseka, joined the United National Party led coalition, the United National Front (UNF) today.
The agreement came following discussions between Mr Fonseka, who was in charge of the army in 2009 when tens of thousands of Tamils are believed to have killed by the Sri Lankan military, and the current UNP leader and prime minister, Ranil Wickremesinghe.
A Memorandum of Understanding was signed between the two parties at Temple Trees on Wednesday.
Mr Fonseka is also to be appointed by the UNP into the ministerial post of the late M.K.A.D.S Gunawardhana.