The death toll was provided to the Times by a source at the Iraqi Ministry of Information. The source also said 41 hostages and 15 security force members were wounded.
Survivors told The Guardian they were about to begin Sunday night services when the gunmen burst into the Sayidat al-Nejat Catholic church. The Times said there were about 100 people in the church, though other media reported lower and higher numbers.
The gunmen seized the hostages after attacking the nearby Baghdad Stock Market earlier Sunday, killing two security personnel and wounding four others, police said.
The gunmen demanded the government release prisoners and said the Christians would be freed in return, police said.
Some of the worshipers escaped before the army moved in, The Guardian reported.
Iraqi authorities ordered the attackers to free the hostages and surrender, warning they would move in, the British newspaper said. A few hours passed as soldiers surrounded the church, including several American units.
Defense Minister Abdul-Kader Jassem al-Obeidi told the Times most of the hostages were killed or wounded when the attackers detonated at least two suicide vests as they stormed the church.
"This was a successful operation with a minimum of casualties, and killing all the terrorists," he said of the siege-ending action, though he added some suspects were arrested.
The Times said its source at the Ministry of Information said eight gunmen suspected of being tied to the Islamic State of Iraq were apprehended.
"It's a horrible scene," Hussain Nahidh, a police officer who saw the interior of the church, told the Times. "More than 50 people were killed. The suicide vests were filled with ball bearings to kill as many people as possible. You can see human flesh everywhere. Flesh was stuck to the top roof of the hall. Many people went to the hospitals without legs and hands."