One of the main suspects in the poisoning of ex-Russian agent Alexander Litvinenko in London has said the former KGB officer may have accidentally killed himself with radioactive polonium.

The statement by Dmitry Kovtun echoed earlier comments made by him and his colleague Andrei Lugovoi. British police have accused the two men of carrying out the killing.

Litvinenko, a former KGB officer, died in 2006 after drinking tea laced with radioactive polonium at a London hotel. On his deathbed, Litvinenko accused Russian president Vladimir Putin of ordering his assassination.

The inquiry in London has paused until July 27 when Kovtun could give evidence via videolink provided he meets certain conditions, including submitting a full witness statement.

Kovtun said at a news conference in Moscow that he will be willing to give evidence if he is given "core participant" status.

Lugovoi has declined to appear, accusing the inquiry of attempting to "whitewash" the involvement of British intelligence in Litvinenko's death.

Kovtun argued that during their meeting at a London restaurant on October 16 Litvinenko refused to eat, saying that he had suffered from poisoning the previous day.

"While entering the now well-known Itsu restaurant, Litvinenko told me that he wouldn't eat anything with us, because the night before he got a really bad food poisoning, he was vomiting as never before and had to call an ambulance," Kovtun said.

"This proves our innocence, it proves that he (Litvinenko) had some contact with polonium probably several days before our first meeting. It remains a mystery how the investigation couldn't have paid attention to this."

Litvinenko's death soured Russia-Britain relations for years, and the investigation into the killing stalled - first because Russia refused to hand over the suspects, then because British authorities would not disclose classified intelligence evidence.

Under the terms of the inquiry, that evidence will be heard in secret, after public hearings end.