London - A British judge investigating the death from radioactive poisoning of former Russian security agent Alexander Litvinenko accused a key suspect on Monday of manipulating the inquiry by agreeing to testify, then refusing at the last minute.Russian businessman Dmitry Kovtun had been due to appear at judge Robert Owen's inquiry by video link from Russia.But inquiry lawyer Robin Tam said that Kovtun now claimed to be bound by obligations of confidentiality to an ongoing Russian investigation.Owen said Kovtun's last-minute change of stance gave rise to "the gravest suspicion that an attempt is being made to manipulate the situation." He has previously warned Kovtun not to try to delay the inquiry, which began in January - more than eight years after Litvinenko's death.The KGB officer-turned-Kremlin critic died in 2006 after drinking tea laced with radioactive polonium-210 at a London hotel. On his deathbed, Litvinenko blamed Russian President Vladimir Putin, and British authorities have also alleged that the Russian state was involved - an accusation Moscow denies.British police have accused Kovtun and another Russian man, Andrei Lugovoi, of carrying out the killing. Both deny involvement, and Russia refuses to extradite them.Lugovoi has declined to co-operate with the British probe, but Kovtun said in March that he was willing to give evidence.He has suggested that Litvinenko may have poisoned himself accidentally while handling radioactive material.Owen gave Kovtun until Tuesday morning to begin his testimony. Otherwise, the inquiry will wrap up this week without him.Ben Emmerson, lawyer for Litvinenko's widow Marina, said he believed there was little chance Kovtun would give evidence."It appears the proceedings are being manipulated in a coordinated way between Mr Kovtun - the murderer - and the Russian state which sent him to commit the murder," Emmerson said.