Doctors treating poisoned Russian focus on objects in his colon

The poison used to attack the former Russian spy Alexander Litvinenko remained unknown last night as doctors searching for a radioactive toxin focused their efforts on pieces of "dense matter" found in his intestines.

The team treating Mr Litvinenko at University College Hospital in central London believe the defector must have knowingly swallowed the three pieces of material which have become lodged in his colon.

X-rays have shown that two of the items are of a similar size to a 2p coin and the other is the shape of a figure "8", according to a hospital source. But it is not yet known whether the three items are linked to Mr Litvinenko's illness, which has left him without a functioning immune system, due to the failure of his bone marrow, and liver damage. Tests are being carried out to identify the pieces of material.

Mr Litvinenko, 43, who believes he is the victim of an assassination attempt sanctioned by Moscow, fell ill three weeks ago after two meetings with contacts in London. He was warned of a death threat against him at one of the meetings.

Doctors said on Tuesday that they now believe thallium sulphate, the "secret agent's poison" originally suspected of causing Mr Litvinenko's condition, is unlikely to be the source.

Instead, they are investigating whether he has ingested a radioactive substance, possibly thallium 201, which is used in hospitals as a "tracer" in cardiac tests.

Friends of Mr Litvinenko claimed that his condition had continued to deteriorate. In a statement, UCH said he remained "unchanged" in a serious by stable condition.

Alex Goldfarb, who helped Mr Litvinenko flee Russia in 2000 after he levelled corruption allegations against the KGB's successor, the FSB, said: "We do not know what has caused the poisoning because the radioactivity has probably left his system... The doctors are supporting his functions and his liver while we wait to see if his bone marrow cells recover."

Mr Litvinenko is believed to have been investigating the murder in Moscow of the dissident journalist Anna Politkovskaya.