Britain is bracing itself for a chill in business relations with Russia over attempts to extradite suspects in the murder of Alexander Litvinenko.

Pressure on the Kremlin to hand over two Russians suspected of conspiring to kill the dissident former FSB officer threatens to put at risk billions of pounds of trade. The arrival in Moscow last night of a business delegation led by Alistair Darling, the Trade and Industry Secretary, underlined the scale of the stakes involved. Mr Darling, who is the first Cabinet Minister to visit Russia since Mr Litvinenko died, said trade between the two countries had trebled in the last five years and was growing annually by more than 20 per cent.

“Our relationship is robust. Where there have been difficulties, we need to talk frankly as partners,” he said.

British officials insist that Whitehall and the Kremlin have been keen to keep the Litvinenko inquiry separate from trade issues. British firms invested $5.5 billion in Russia in the first nine months of 2006 and Russian companies raised $15 billion with share offerings on the London Stock Exchange.

But they acknowledge that this would change if detectives produced evidence of involvement by Russian security services in the poisoning of Mr Litvinenko in London with radioactive polonium-210. Moscow insists that it will not extradite Andrei Lugovoy, a former KGB guard, and his business partner Dmitri Kovtun, pointing to a ban in Russia’s constitution on the trial of citizens abroad.

The Crown Prosecution Service is considering a file submitted by Scotland Yard. Mr Lugovoy and Mr Kovtun have repeatedly protested their innocence, but sources said yesterday that detectives believe sufficient evidence exists to charge them with conspiracy to murder Mr Litvinenko.

Russian prosecutors plan to travel to London to interview more than 100 people they believe are linked to the case.

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