The declaration was supported by 96 members of the council, with three voting against and another three abstaining.
The Labor Party's council decided to open merger talks with the Order and Justice party earlier on Sunday.
The two leaders – MEP Rolandas Paksas of the Order and Justice Party and MP Viktor Uspaskich of the Labor Party – had last week expressed support to the merger plans.
They expressed belief that the merged party would become the strongest center party in Lithuania.
Should the two parties merge, their new political group in parliament would be the biggest with 39 seats in Lithuania's 141-seat parliament. The ruling Social Democratic Party currently has 38 members in the Seimas.
The Labor Party and its leader Uspaskich along with Parliamentary Vice-Speaker Vytautas Gapšys and MP Vitalija Vonžutaitė are standing trial in Vilnius court on double bookkeeping charges.
Prosecutor Saulius Verseckas in charge of the case has said that, in case of merger, the Labor Party's chances of escaping criminal liability would depend on specific terms and conditions of the merger and the agreement to be signed by the two merging parties.
