Russian President Vladimir Putin has said the route of a controversial new oil pipeline should be altered to avoid the world's deepest freshwater lake.
Ecological campaigners have fought the Lake Baikal route
Mr Putin said the route, which would have passed close to Lake Baikal, should move 40km (25 miles) north.
Environmentalists who feared the eastern pipeline would pose a risk to Lake Baikal's unique biodiversity say the decision is a campaign victory.
The pipeline will link the Siberian oil fields and the Pacific coast.
Lake Baikal, designated a World heritage site by Unesco, is home to hundreds of unique species and the region includes the last remaining wild habitat of Russia's Amur leopard.
The area is also prone to seismic activity and campaigners feared earthquakes could disrupt the pipeline, causing an oil spill and irreversible damage to the wildlife.
The state-backed pipeline monopoly Transneft has in the past said that rerouting the 1.6 million-barrel-a-day pipeline from the proposed path along the shores of Lake Baikal would cost up to a billion dollars.
Mr Putin's orders go against recommendations from the state environment watchdog, which had backed Transneft's plans.
But during a meeting on Siberian development, which included Transneft directors, Mr Putin supported recommendations from the deputy head of Russia's Academy of Sciences, Nikolai Laverov, that the route be changed.
Mr Putin said construction could start at either end to give time for studies to be carried out to decide on a new route north of Lake Baikal, according to the Itar-Tass news agency.
"The route should go north of the area that Laverov has named," Mr Putin insisted.
"In this way we are significantly reducing the ecological risks before construction begins. We can consider this agreement final."
Greenpeace Russia, which has led demonstrations against Transneft's proposed route, has welcomed Mr Putin's decision.
Spokesman Andrey Petrov said: "It is victory for the whole of civil society in Russia, not just the ecological movement."
He said environmental considerations would still have to be taken into account when the new route was planned, but moving it from the shore of Lake Baikal had been a key issue.