Ukraine's parliament has rejected constitutional changes that the opposition said would have weakened the powers of any future president.
The proposals have prompted rowdy parliamentary protests
The proposed reforms were criticised by the West and led to rowdy protests in parliament by opposition members.
President Leonid Kuchma had backed the changes but the bill fell six votes short of the required 300 votes.
The opposition believes it is in a strong position to win the presidential elections in October.
The BBC's Helen Fawkes in Kiev says it is one of the most contentious political issues faced by Ukraine's parliament since its independence in 1991.
The reforms were given initial approval by parliament in December, despite noisy protests by the opposition, who let off police sirens, sang songs and physically blocked the rostrum inside the parliamentary chamber.
Opposition claims victory
The opposition said the proposed changes would weaken the power of the next president.
But supporters of President Leonid Kuchma said the reforms were needed to establish an effective and responsible administration.
Opposition MPs said the vote was a victory for their leader, former Prime Minister Viktor Yushchenko, and increased his chances in the presidential election.
Our correspondent says the most controversial amendment, which allows parliament instead of the voters to choose the president, was dropped after the pro-presidential majority bowed to pressure from Europe, the US and the opposition.
But there is still concern that the constitution could be changed just months before the election.