British investigators believe that Alexander Litvinenko’s killers used more than $10 million of polonium-210 to poison him. Preliminary findings from the post mortem examination on the former KGB spy suggest that he was given more than ten times the lethal dose.

Police do not know why the assassins used so much of the polonium-210, and are investigating whether the poison was part of a consignment to be sold on the black market.

They believe that whoever orchestrated the plot knew of its effects, but are unsure whether the massive amount was used to send a message — it made it easier for British scientists to detect — or is evidence of a clumsy operation.

A British security source said yesterday: “You can’t buy this much off the internet or steal it from a laboratory without raising an alarm so the only two plausible explanations for the source are that it was obtained from a nuclear reactor or very well connected black market smugglers.”

Alexander Goldfarb, a friend of Litvinenko, said: “Only a state-sponsored organisation could obtain such a large amount of polonium-210 without raising suspicion on the international market.”

In Moscow, Scotland Yard detectives have asked to question further two Russian businessmen who met Litvinenko several times in the fortnight before he died, including November 1, the day he fell ill. Both men, Andrei Lugovoy and Dmitri Kovtun, were contaminated with polonium-210 and remain isolated in a clinic.

The men — who have been friends since they were 12 and attended the same Moscow military academy — deny any role in the poisoning and claim that they are victims. German police, however, have begun a criminal investigation into Mr Kovtun after traces of polonium-210 were found at severallocations he visited in Hamburg. Neither man has explained why the radioactive poison was discovered in London in places they visited as long ago as October 16.

The nine British detectives sent to Moscow to investigate the murder are likely to return home this week. Russian authorities have blocked their inquiries and left them on the sidelines as their own officials question the main figures in the investigation.

Security sources told The Times that Russian officials refused to ask Mr Kovtun and Mr Lugovoy questions to which the British team wanted answers. Aware of the diplomatic sensitivities of this case, police chiefs and politicians have avoided any public disagreement with Russia or criticised the way Yuri Chaika, the country’s Prosecutor-General, has effectively hijacked the investigation.

United Nuclear Scientific Supplies of New Mexico, one of the few companies licensed to sell polonium-210 isotopes online, said that as a single unit costed about $69, it would take at least 15,000 orders, costing more than $10 million, to kill someone.

The company said that as it sold to only a handful of outlets in the United States every three months, anyone placing an order for 15,000 units would be spotted.

Experts reckon that as little as 0.1 micrograms of polonium-210 would be enough to kill — the equivalent of a single aspirin tablet divided into 10 million pieces.

The killers would also have to know that polonium-210 decays rapidly; its half-life is only 138 days.

The first consignment is reported to have arrived in the second part of October. The rest arrived in two further batches but police do not know why the couriers risked smuggling further supplies into Britain when the original amount was sufficent to murder their target.

The latest theory, made by Alexander Shvets, another former KGB spy, is that Litvinenko uncovered damaging information about a powerful Russian businessman with close links to President Putin.

In Moscow yesterday, a vigil was held for the more than 200 journalists who have died violently since the fall of the Soviet Union.

Journalists held photographs of many of the victims, including Anna Politkovskaya, whose murder Litvinenko claims he was investigating when he was murdered.