Unipetrol has offered to sell its controlling stake in the Czech Republic's biggest refinery to Royal Dutch/Shell Group, Conoco and other companies that now own the rest of the unit's shares. Unipetrol offered its fifty one percent of Ceska Rafinerska to Agip Petroli International, Conoco Central and Eastern Europe Holdings and Shell Overseas Investments, who between them own the rest of the unit, Unipetrol spokesman Tomas Zikmund said. The petrochemicals holding company declined to comment on the price it is asking for Rafinerska, which analysts say is Unipetrol's most valuable asset. Unipetrol must make the offer under a 1995 agreement that gave the international oil companies the right to buy the rest of Rafinerska when the government relinquishes its control over it. The Czech government in February agreed to sell its majority stake in Unipetrol to Czech-Swiss company Agrofert. Conoco, the third-biggest US oil company, backed the Agrofert bid. The Czech Anti Monopoly Office has not yet approved the transaction however.