Traces of the radiation that killed the former Russian spy Alexander Litvinenko has been found at more addresses in London.

Officers are believed to be searching a West End office block where the ex-KGB officer photocopied a hitlist.

It comes as three people have been taken up in a specialist radiological clinic after being in contact with Mr Litvinenko at the Millennium Hotel in Grosvenor Square or Itsu sushi bar in Piccadilly.

The 43-year-old Russian, who died last Thursday, fell ill on November 1 after visits to both places.

Police have since found traces of the deadly POlonium-210 at the two venues, as well as at the former spy's home in Muswell Hill, north London.

Over 450 people have called an NHS hotline to check if they too had been contaminated.

Of them, 18 have been asked to submit urine samples, with the three being referred to the clinic as a precaution.

Mr Litvinenko, an outspoken critic of Vladimir Putin who fled to Britain, claimed just before his death that the Russian president had him poisoned.

Scotland Yard confirmed they were now treating his death as "suspicious" rather than "unexplained", but officers have not yet opened a murder inquiry.

The Health Protection Agnecy (HPA) confirmed tests for radioactivity were underway "at a number of locations that Mr Litvinenko may have visited".

These are thought to include an office block where Mr Litvinenko photocopied a hitlist he got from Mario Scaramella, the Italian environmental professor with whom he had lunch at Itsu.

POLONIUM-210 FACTS

  • Tiny amounts occur naturally in the environment and in people
  • Emits Alpha radiation, which travels only a few inches and is stopped by the body's dead layer of outer skin
  • But if high doses enter the body, it is very dangerous
  • Damages soft tissue and organs

The list named the targets of an ex-KGB death squad who wanted to eliminate Putin's critics. Both Scaramella and Litvinenko were on the list.

A post-mortem on Mr Livionenko's body has been delayed while experts assessed whether it was safe to perform the procedure.

The inquest into his death will open at St Pancras Coroner's Court on Thursday. The hearing is expected to be short, with the coroner receiving proof of Mr Litvinenko's identity.

The government's Cobra committee, which brings together ministers, officials and experts in emergencies, has met several times to assess the public risk. Home secretary John Reid will chair another Cobra meeting tomorrow.

Today Itsu remained boarded up for decontamination, while parts of the Millennium Hotel remained sealed off under strong police guard.