"The cause is evident," Luzhkov told journalists in Tokyo. "Now, after investigators have completed their work, I can be certain that this manmade disaster was caused by improper welding carried out in 1976."
The Investigative Committee earlier ruled out the possibility of the incident being a terrorist act.
The mayor said gas supplies will soon resume in full.
Yevgeny Bobylev, a spokesman for the Moscow branch of the Russian Emergencies Ministry, said investigators would analyze data collected during the probe at the accident site to decide whether a criminal case should be launched.
"The results of the probe have been handed to the western office of the Moscow investigation department," he said.
The gas pipeline on Ozernaya Street in west Moscow exploded early on Sunday shortly after Russia finished celebrating Victory Day, sending flames up to 300 meters into the air. Five people were treated for burns and some 14 cars were set ablaze.
The fire, acknowledged as Moscow's largest since World War II, was put out 15 hours after it started.
Some 100,000 Muscovites were left without telephone and Internet services until Monday, after the fire destroyed or damaged telephone cables. One thermal power plant was stopped due to reduced gas supplies.