Ukrainian President Viktor Yushchenko's party plans to reunite with its estranged "Orange Revolution" partners to form a governing coalition.
Yushchenko and Tymoshenko were at the centre of a power struggle
A spokeswoman for Mr Yushchenko's Our Ukraine bloc said its political council had agreed to team up with ex-PM Yulia Tymoshenko's party and the Socialists.
Mr Yushchenko sacked Ms Tymoshenko as prime minister last September.
Ms Tymoshenko's key demand was to return as prime minister. It is not yet clear if she will get the job again.
Mr Yushchenko's party came third in the general election held on 26 March, while Ms Tymoshenko's bloc came second.
The two pro-Western liberal leaders had stood together before huge crowds of supporters in Kiev during the 2004 "Orange Revolution" - the huge street protests which swept them into power.
But their alliance collapsed last year amid bitter rivalry between Ms Tymoshenko and some of Mr Yushchenko's top aides.
Talks will now be held to agree on an action plan for the new coalition.
The pro-Russian opposition party led by Viktor Yanukovych came first in last month's parliamentary election, but did not get enough votes for an outright victory.
Mr Yushchenko's Our Ukraine party ruled out including Mr Yanukovych's Party of the Regions in the new coalition.
Mr Yanukovych was declared the winner of the presidential election in November 2004, but allegations of widespread vote-rigging sent hundreds of thousands of Ukrainians out onto the streets to demand change.
In the Orange Revolution - named after Mr Yushchenko's campaign colour - the election result was overturned and Mr Yushchenko went on to win a re-run.