Leading article: Diplomatic interests must not obstruct this murder investigation

The Litvinenko affair, as we must now call it, grows curiouser and curiouser. In London the Itsu sushi bar in Piccadilly has now been given a clean bill of health. The spotlight has shifted to the Millennium Hotel in Mayfair, where seven members of the bar staff have been affected by radiation. The bar is the place where Alexander Litvinenko is believed to have met two Russians on 1 November. After a spell under observation, the talkative Italian, Mario Scaramella, has been discharged from hospital.

Meanwhile in Moscow, British detectives are trying to interview the two Russians who drank with Mr Litvinenko at that possibly fatal meeting. The one they managed to see was reported next day to be in a coma, suffering radiation sickness or absolutely fine - take your pick. The other, Andrei Lugovoi - who seems to have been a regular visitor to London through the autumn - is in the same Moscow hospital, perhaps suffering the effects of radiation, perhaps not. He may or may not have been prepared to speak to the Met's finest yesterday.

Back in London, the former spy was laid to rest in the cemetery where Karl Marx is buried, his sealed coffin borne by the elite of Russia's self-appointed opposition in exile. But mystery pursued him to the grave. His father believed that he had converted to Islam. A ceremony in his name took place at Regent's Park Mosque. The helter-skelter of events between the first disclosures about Mr Litvinenko's illness and his interment is improbable even by the most far-fetched standards of Cold War espionage. Planes were grounded in London and Moscow; air passengers around the globe were warned that they might have been exposed to radiation. A former Russian prime minister was rushed home from Ireland, apparently poisoned - but not, he later insisted, by President Putin. Traces of radiation were detected as far apart as the British embassy in Moscow and Arsenal's Emirates stadium. and we are all experts on polonium-210 now.

Rip-roaring stuff all this may be. But it is important not to lose sight of two exceptionally serious aspects. The first is that an outspoken foe of Russia's President has been deliberately killed by a lethal dose of radiation in London. The authorities in London and Moscow have a duty to do their utmost to find the culprit. That the Russian authorities have undertaken to assist the investigation is a positive sign. They must be kept to their word.

The second relates to British-Russian relations. Russia's foreign minister, Sergei Lavrov - an adept and experienced diplomat - has warned of the damage that politicising the affair would do to bilateral relations. Fear of where the findings might lead, however, must not be allowed to inhibit the British investigation. Ministers have so far been wisely cautious in their statements. They have declined to speculate about where the blame may lie, deferring to the police. The speculative running has been made by sections of the British media and certain members of the recent Russian emigration, who have Mr Putin in their sights. Most of the Moscow media, needless to say, favour quite a different conclusion. A documentary fiercely critical of Britain was aired on Russian state television a couple of nights ago.

It would be wrong to indict the Kremlin without justification. If evidence is found that points in that direction, however, there must be no soft-pedalling because of our need for Russian energy or fears about the possibly destabilising effect on the position of Mr Putin. There need be no worries then about the potential damage of the Litvinenko affair to Britain-Russia relations. The damage will already have been done - and the Kremlin will have only itself to blame.