It will unite Yanukovych's Party of Regions, the Socialist Party, and Yushchenko's Our Ukraine. The Communists, who did not sign the national unity declaration, are likely to be excluded.
Yushchenko says the declaration is intended to safeguard Ukraine's move toward greater integration with Western Europe and free-market reforms.
A photo gallery of political developments in Ukraine since the March 26 elections (Flash required)
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The development follows Yushchenko's announcement earlier today that he had nominated Yanukovych as prime minister. Parliament is expected to confirm his nomination later today.
For Yanukovych, the vote -- considered a foregone conclusion -- will complete a remarkable comeback.
Humiliated by the Orange Revolution in 2004, the pro-Russian Yanukovych was written off by his opponents.
But Yushchenko has been forced to swallow his pride and nominate the man he defeated for the presidency.
The alternative was the dissolution of parliament, fresh elections, and the possibility of more chaos for a country that has been without a government for four months.