The Our Ukraine leader Vyacheslav Kyrylenko resigned following Monday's session at which the bloc decided to sign the deal despite opposition from the president. The move was backed by 37 of 72 bloc members.
Without the support of 35 members of Our Ukraine, who rejected the deal, the coalition will only have 214 seats in parliament, short of the 226 required for a majority.
The revival of the coalition of Our Ukraine and the groups led by Prime Minister Yulia Tymoshenko and parliamentary speaker Volodymyr Lytvyn - which collapsed in September after months of political backbiting - was announced last week.
Earlier reports said Our Ukraine Deputy leader Borys Tarasyuk had been authorized to sign the deal instead of Kyrylenko.
Tarasyuk confirmed Kyrylenko's resignation at a parliamentary session earlier on Tuesday. "A new leadership will be elected, which will not heed orders from the outside," he said.
The bloc members, who met with President Viktor Yushchenko on Monday, said he had not approved of the idea of joining the coalition. He had initially backed the reestablishment of the coalition "on condition of transparency."
Yushchenko's Our Ukraine and Tymoshenko's bloc has argued over a host of differences, including Russia's war with Georgia in August. Yushchenko dissolved parliament in September and called snap elections, which were subsequently called off after Ukraine's economy was devastated by the global credit crunch.
Yushchenko and Tymoshenko were allies in the 2004 "orange" revolution that swept them to power, but have since drifted apart. Both are expected to run for president next year.
In the agreement, the resurgent coalition repeated its opposition to early polls.
