Bogoljub Karic is at the centre of a major corruption inquiry
Serbian authorities have issued an arrest warrant for Sreten Karic, the brother of a tycoon whose Mobtel mobile phone company is under investigation.
Sreten Karic had failed to report to police for questioning, Serbia's B92 news website reported. He is accused of fraud and tax evasion.
His brother Bogoljub is involved in a bitter row with the government over Mobtel's business practices.
Bogoljub heads an opposition party called the Strength of Serbia Movement.
In December, the government revoked a licence for Mobtel, accusing the Karics of endangering Serbia's security by signing a deal with a Kosovo company, run by an ethnic Albanian.
The Karics are among Serbia's richest families.
Bogoljub has dismissed the Mobtel allegations as a tale concocted by the authorities to thwart his political party, B92 reports.
He said his party had achieved "a very high rating" and so "the public's eyes must be blinded with a fictitious scandal regarding Mobtel and other rumours of theft, robbery and business crimes".
Sreten Karic is accused of tax fraud in connection with the sale of apartments to associates.
He and another Mobtel director, Olga Zilovic, allegedly avoided paying 2.8m euros ($3.4m) in taxes.
B92 reports that the authorities are planning to arrest and investigate many current and former managers of Mobtel.
Bogoljub appeared last week in UN-administered Kosovo. His party has been accused of trying to bribe an MP in a rival party to change sides.Mobtel has been placed temporarily in the administration of Serbia's other mobile operator, MTS, which is run by state-owned Telekom Serbia.