Yingluck Shinawatra says she is still confident of becoming Thailand's first female prime minister despite an obstacle thrown up by the country's election commission.

Ms. Yingluck was among 142 of the 500 winning candidates in this month's election whose victories were not certified by the commission late Tuesday because of complaints of fraud or other issues. But she said Wednesday she remains confident that the commission will treat her and her Pheu Thai party with justice.

The commission also failed to certify the re-election of outgoing Prime Minister Abhisit Vejjajiva, who faces complaints of vote-buying. He was returned to parliament personally despite his party's defeat.

The commission did not specify the complaints against Ms. Yingluck, whose party won an outright majority with 265 seats and plans to rule in a coalition with several smaller parties. The supervisory body must now review her case and those of the other remaining candidates.

The commission is required to endorse at least 475 of the 500 candidates in time for the new parliament to begin its work in early August.

Any candidates whose election is not ultimately certified will have their cases referred to the Constitution Court for a decision on whether they should be disqualified. But in the past, the election commission has never failed to certify top party-list candidates.

The commission has already cleared Ms. Yingluck on a charge that she tried to buy votes by cooking noodles and distributing them to voters during a campaign appearance. She is the younger sister of former Prime Minister Thaksin Shinawatra, who was overthrown in a 2006 coup and now lives in exile to avoid a conviction on corruption charges.