Having tried to pacify the Russian bear over the nuclear missile row by declaring that it was "not the enemy", George W Bush last night provoked it still further by telling Vladimir Putin that he had "derailed" democracy in his country.

In a speech in Prague to democracy and human rights activists, the US President acknowledged free societies emerged "at different speeds in different places". He then noted: "In Russia, reforms that once promised to empower citizens have been derailed, with troubling implications for democratic development."

In apparent anticipation of a backlash from Moscow, Mr Bush insisted the fact that he could criticise Russia showed the strength of relations between the two countries. "America can maintain a friendship and push a nation toward democracy at the same time," he said.

The US President also criticised China, saying: "China's leaders believe they can continue to open the nation's economy without also opening its political system."

China will be represented at today's G8 whose members are Britain, Canada, France, Germany, Italy, Japan, Russia and the United States.

Last night, Sergei Lavrov, the Russian Federation's Foreign Minister, said Washington's insistence the missile shield would guard against attack from rogue states such as Iran and North Korea did not reflect military reality.

At an earlier press conference in the Czech capital, Mr Bush sought to calm fevered brows over the nuclear missile dispute by insisting the Cold War was over and Russia was no longer the enemy.

He declared: "The people of the Czech Republic don't have to choose between being a friend of the United States or a friend with Russia. You can be both. We don't believe in a zero-sum world."

The US plan for a missile shield could lead to a radar system being built in the Czech Republic and interceptor missiles being placed in Poland.

Washington insists the shield will be "purely defensive" and aimed at knocking out rockets from rogue states. However, Moscow insists no such threat exists and fears the missiles will be pointed at Russia. Mr Putin has warned if the shield is built, then "retaliatory steps" will be taken with the possibility of Russian rockets targeting European cities.

Mr Bush sought to allay fears, saying: "My message will be Vladimir - I call him Vladimir - that you shouldn't fear a missile defence system. As a matter of fact, why don't you co-operate with us on a missile defence system? Why don't you participate with the United States? Send your generals over to see how such a system would work, send your scientists."

The US missile shield plan has not gone down well with other countries too. Iran has labelled it "a joke", pointing out its own missiles cannot even reach Europe let alone America. China said it was "not conducive to mutual trust of major nations and regional security".

As world leaders gather on the Baltic coast today for their three days of talks, Angela Merkel, the German Chancellor hosting the event, will seek to ensure the political focus is on the key agenda issues of climate change and aid to Africa.

While Downing Street stressed the nuclear missiles row would "not overshadow" other subjects on the agenda, it again made clear Moscow had a choice to make.

"It is up to Russia to decide what sort of relationship it wants. We want to be constructive but part of being constructive means sharing the same values, both politically and economically," said Tony Blair's spokesman.

Britian is also at odds with Russia over the murder of ex-spy Alexander Litvinenko.

Yesterday, the widow of Mr Litvinenko called on G8 leaders to put pressure on Mr Putin over her husband's murder.

Marina Litvinenko called on leaders to drive home the case for the extradition to Britain of former KGB agent Andrei Lugovoy, accused of poisoning her husband with radioactive Polonium-210.

Mrs Litvinenko also accused the Russian leader of attempting to "cover up" the killing and said she was shocked at his "defiance" of the British extradition bid.

Mr Putin said efforts to extradite Mr Lugovoy were "pure foolishness" and described the British prosecutors' request as "PR and politics".

British prosecutors want to charge Lugovoy with murdering Mr Litvinenko, 43, in London last November.