AP quoted an unidentified commission official as saying the Yulia Tymoshenko Bloc was refused registration but gave no other details.
The top 10 candidates on the bloc's list are all former legislators.
Speaking at a press conference in Kyiv today, Tymoshenko called the move illegal and said that the group will appeal the decision.
The official campaign period began on August 2 for the polling, which will be held on September 30.
Candidates from Prime Minister Viktor Yanukovych's party and from President Viktor Yushchenko's Our Ukraine bloc were successful in registering with the commission.
Eight of the commission's 15 members represent the pro-Russian governing coalition led by Yanukovych, whose bid for the presidency was thwarted by the Orange Revolution that broke out after tainted elections in late 2004.
Tymoshenko was a leader of that bloodless revolution, along with current Yushchenko.
She also served as prime minister in 2005 ahead of a high-profile dispute with Yushchenko.