Ukrainian President Viktor Yushchenko has rejected a proposal to appoint his pro-Russian rival, Viktor Yanukovych, to the post of prime minister.
Scuffles broke out among MPs in the Ukrainian parliament this week
He said the forces in parliament which put forward Mr Yanukovych's name had violated the constitution when they formed a coalition last week.
His message coincided with another day of chaos in the parliament.
Ukraine has not had a government since March, and Mr Yushchenko has warned he may have to call new elections.
Ukraine's pro-Western parties had been on the verge of forming a government last week - after three months of negotiating - when the Socialist party broke ranks and joined forces with Mr Yanukovych's Party of the Regions, and the Communist Party.
The new coalition controls just over half the seats in the 450-seat parliament.
Mr Yushchenko said in a letter to parliament on Thursday that the coalition had been formed too quickly.
In the chamber itself, pro-Western parties drowned proceedings with sirens and loudhailers.
On Tuesday fights broke out between a number of rival MPs.
Mr Yanukovych was the loser in the presidential ballot in 2004 - the election which was marred by claims of vote-rigging and sparked the mass protests at the Orange Revolution.
It was these demonstrations which ushered the pro-Western Mr Yushchenko to power.
However Mr Yanukovych's party, which has strong support in the east of the country, won more votes than any other in parliamentary elections held in March.
Mr Yushchenko urged politicians on Wednesday to form a government capable of consolidating Ukraine.
Correspondents say Mr Yanukovych has been trying to persuade Mr Yushchenko's Our Ukraine party to join his party in a loose coalition.
They say Mr Yushchenko may not have ruled this out, but would draw the line at making Mr Yanukovych prime minister.