Liana Askerova, a restaurant owner, is believed to be a link between the paymaster in the September 13 shooting of Kozlov, who led a crusade against money laundering and other criminal practices in the banking sector, and the contract killers. She was arrested in mid-January.
"The Basmanny [district] court's decision remains unchanged, and the appeal has been turned down," the judge said.
The defense insisted that Askerova be released, citing her poor health. "Askerova is incurably ill, her health is growing worse, and she needs in-patient care," said Nikolai Klen, who represents Askerova's interests.
The businesswoman, who participated in the court hearings via a video link, pleaded not guilty. "I have nothing to do with Kozlov's death, and I am not going to evade the investigation," she said.
However, the court sided with the prosecution.
Askerova is said to have identified a commercial banker, Alexei Frenkel, as the mastermind behind Kozlov's assassination.
Frenkel's bank was among those closed down by Kozlov as part of his efforts to clean Russia's financial sector of corruption.
Following his arrest, Frenkel, who could face life in prison, published two letters accusing the Central Bank of accepting bribes in exchange for allowing banks to join the deposit insurance system. Prosecutors are investigating Frenkel's accusations.
"During the questioning of my client, Alexei Frenkel, an investigator told us the inquiry would be completed within a week," Frenkel's lawyer Igor Trunov said last week. The Prosecutor General's Office said in February that the probe would continue into mid-May.
Kozlov, 41, was shot in the head late September 13 in what appears to have been a contract killing. Kozlov and his driver, who was killed instantly, were gunned down as they left a sports stadium in northeast Moscow. The banker underwent emergency surgery, but died several hours later.
In all, seven suspects are now in custody for their alleged involvement in Kozlov's murder.
