A statement, signed by Foreign Secretary David Miliband and the foreign ministers of the the US, Canada, France, Germany, Italy and Japan said it "deplored" Moscow's "excessive use of military force" in Georgia.
The move, described as an "unprecedented step" by the Foreign Office, followed a warning from Mr Miliband to Russian president Dmitry Medvedev that he had a "big responsibility" not to provoke a new Cold War.
The foreign ministers said Moscow's recognition of South Ossetia and Abkhazia violated Georgia's integrity and sovereignty.
It said: "We ... condemn the action of our fellow G8 member.
"Russia's recognition of the independence of South Ossetia and Abkhazia violates the territorial integrity and sovereignty of Georgia and is contrary to UN Security Council resolutions supported by Russia."
It went on: "We deplore Russia's excessive use of military force in Georgia and its continued occupation of parts of Georgia."
The hard-hitting statement followed comments by Mr Miliband, who visited the Ukrainian capital Kiev yesterday, that that sight of Russian tanks on the streets of Georgia had come as a "rude awakening" after so many years of peace in Europe.
He accused Mr Medvedev of trying to "redraw the map" of the Caucasus in a way that threatened the stability of the entire region through his unilateral recognition of the breakaway provinces of South Ossetia and Abkhazia.
"The Russian President says he is not afraid of a new Cold War. We don't want one. He has a big responsibility not to start one," he said in a speech to university students.
He called for a new policy of "hard-headed engagement" with the Kremlin, while rejecting "knee jerk" demands to break off relations between the West and Moscow or expel Russia from the G8.
Following talks with President Viktor Yushchenko, Mr Miliband said that he had come to Ukraine to demonstrate Britain's support for the former Soviet republic amid fears in the West that it could be the next target of Russian aggression.
Like Georgia, the pro-Western Ukrainian government wants to join Nato and the EU - moves fiercely opposed by the Kremlin.
"In the midst of the Georgia crisis, I want to re-affirm the commitment of the United Kingdom to support the democratic choices of the Ukrainian people," Mr Miliband said."
He said that the West had to be prepared to take a stand in the face of Russian aggression in Georgia.
"The Georgia crisis has provided a rude awakening. The sight of Russian tanks in a neighbouring country on the 40th anniversary of the crushing of the Prague Spring has shown that the temptations of power politics remain," he said.
He made clear that the West would not allow the Russians a "veto" on countries like Georgia and Ukraine joining Nato.
However he said that it was impractical to try to isolate Moscow internationally, and that instead the West should be prepared to take a tougher line on issues like energy negotiations, while reducing its dependence on Russian gas.
He insisted that Moscow was paying a price, politically and economically, for its actions in Georgia.
"Today Russia is more isolated, less trusted and less respected than two weeks ago. It has made military gains in the short term. But over time it will feel the economic and political losses," he said.
"If she truly wants respect and influence, and the benefits which flow from it, then Russia needs to change course."
Tory leader David Cameron welcomed Mr Miliband's visit to Kiev but said that his speech did not go far enough.
"I think that the Government should be looking at all of the potential mechanisms, all of the things in the locker that they have," he told Sky News.
"There is a danger of having your cake and eating it, of saying that you want a tough response to Russian aggression, you want to build a strong coalition, but then ruling out so many of the potential items you can choose to actually say 'Hang on, this is not a way to behave."'
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