By Alex Kirby
BBC News Online environment correspondent
Russia says it will not ratify in its present form the Kyoto Protocol designed to mitigate global warming.
Italian environment minister Altero Matteoli (L) opened the Milan climate conference
"The Kyoto Protocol places significant limitations on the economic growth of Russia," presidential aide Andrei Illarionov has announced in Moscow.
The landmark environmental pact cannot now enter into legal force, especially since the US has also repudiated it.
It means the protocol will either have to be renegotiated or the nations that have signed will have to go it alone.
The Russian decision will come as a devastating blow to many of the delegates at a meeting of the signatories to the United Nations Climate Change Convention, being held in Milan this week.
But they will not be without all hope. Michael Williams, a spokesman for the UN Framework Convention on Climate Change, said he hoped the Russians could still be won round.
"Russia has over the last few months sent mixed signals but [President Vladimir] Putin didn't rule out ratification last September," he told the AFP news agency. "We remain optimistic that Russia will indeed ratify."
'Vital' first step
The protocol requires industrialised countries to cut their emissions of six gases which scientists believe are exacerbating natural climate change.
Signatories will by some time between 2008 and 2012 have to cut emissions to 5.2% below their 1990 levels.
But many scientists say cuts of around 60-70% will be needed by mid-century to avoid runaway climate change.
The convention's executive secretary, Ms Joke Waller-Hunter, told BBC News Online: "It's wrong to think the protocol will do so little that it's insignificant.
"It's a very important first step that can lead to much more far-reaching measures. Yes, it's a peanut - but a vital one in the long run."
The protocol would have entered into force when 55 signatories had ratified it, including industrialised countries responsible for 55% of the developed world's carbon dioxide (CO2) emissions in 1990.
Some critics say President Bush's decision that the US, which emits more greenhouse gases than any other country, would not ratify the protocol had already condemned it to irrelevance.
But enough other signatories have done so for it to have entered into force if Russia, another big polluter, had decided to ratify.
The protocol's supporters question Mr Illarionov's view that ratifying Kyoto would harm Russia's economic prospects.
President Putin at a climate conference in September said he wanted more from Kyoto
Each country's emission reduction targets were set in 1990, when the Soviet Union's heavy industries were still pumping out huge amounts of pollution.
Russia (and most of the states of the former Soviet bloc) cannot now afford either the industries, or even the fuel that drove them.
So Moscow has the notional right, under the protocol, to emit vast quantities of "paper" pollution which will never leave a factory chimney.
What it can do is sell its unused emission entitlements to industrialised countries which are close to exceeding their own allowances, and this could be a very lucrative trade.
Throughout first the US and now Russian refusal to ratify the protocol, the European Union has remained an enthusiastic advocate of its potential.
The EU Environment Commissioner, Margot Wallstrom, commenting on an announcement by the European Environment Agency that the EU was on course to miss its targeted emission cuts, remained resolutely hopeful.
"The Kyoto Protocol is not dead", she said. "It has maybe held its breath for a little while as we are all waiting for the Russian ratification."
A few minutes later came Mr Illiarionov's bombshell, tempered only slightly by his qualification that Russia would not ratify the protocol "in its present form".
One of the criticisms of Kyoto made by the US and others is that it requires only industrialised countries to make emissions cuts.
The developing countries are exempt for the moment, though negotiations were due to start soon on the cuts they will soon have to make.
Many of them are already working to live up to the spirit of the protocol. But if neither the US nor Russia is willing to sign up to the letter, the poor world may lose interest as well.