Vladimir Nekrasov, who owns the Arbat Prestige cosmetics chain, was detained in Moscow along with Sergei Shnaider, 62, an alleged mafia boss also known as Semyon Mogilevich, in late January.
"The need to extend custody for my client arose due to the large number of investigation activities that have to be held, as well as the need to receive the results of an examination, which in my opinion was held in violation of the law," Alexander Pogonchenkov, a lawyer for Shnaider, said.
He said his client continues to suffer from poor health.
In early June, the lawyer requested the release of his client on health grounds, saying he had suffered a heart attack and was being deprived of essential medical treatment.
In April, the Moscow City Court had refused to release Shnaider despite an offer of a $2 million bail payment, equal to the back tax claim against Arbat Prestige. The court ruled that the suspect must remain in custody until June 23 pending the investigation.
Shnaider has been wanted by a number of countries, including the United States, for more than 15 years for involvement in organized crime and fraud. Believed to control the largest Russian mafia syndicate in the world, he is suspected of being involved in arms dealing, drug trafficking, the sex trade, and money laundering.
He served two jail terms in the Soviet Union in the 1970s for currency dealing offenses and fraud.
Russian authorities allege that he was behind a tax evasion scheme said to have been used by Arbat Prestige.
Arbat Prestige was established in 1989 and opened its first retail outlet in 1998. It currently runs a network of 64 outlets across Russia.
