BERLIN, April 10 (RIA Novosti) - A key witness in the murder of security service defector Alexander Litvinenko said Tuesday that German prosecutors have refused to ensure his guaranteed return to Russia if he decides to travel to Germany for questioning.

Russian businessman and former security service agent Dmitry Kovtun said in an interview with Hamburger Abendblatt daily that he could be detained in Germany if he travels there for questioning, and even extradited to the U.K. where he is considered the main suspect in the murder investigation.

"My lawyer informed the [German] prosecutors that I am willing to travel to Germany if they guarantee my safe return [to Russia]," the paper quoted Kovtun as saying.

"Unfortunately, that proposal has been rejected," Kovtun said.

Litvinenko, an outspoken Kremlin critic with close ties to Berezovsky, defected in 2000 and received a British passport shortly before his death in London in November of last year. Doctors said traces of radioactive Polonium-210 were found in his body.

Kovtun and another former security officer, Andrei Lugovoi, met with Litvinenko in a London hotel shortly before he was hospitalized with symptoms of poisoning, and have themselves undergone radiation checks. Both have tested positive for the presence of the deadly radioactive element in their blood.

In December of last year, police in the German city of Hamburg found traces of Polonium-210 in locations visited by Kovtun before his trip to London, and launched a criminal investigation into his possible polonium smuggling.

The agent-turned-businessman has been questioned by investigators probing the case several times. He has repeatedly denied any involvement in Litvinenko's death, insisting that he is himself a victim.

Kovtun was hospitalized in December of last year with symptoms of radioactive poisoning, and is still undergoing treatment and tests for radiation at a Moscow hospital every two weeks.

More than 700 people in the U.K., Russia and Germany have been tested for Polonium-210 after British forensic doctors confirmed the substance had been the cause of Litvinenko's death. The tests revealed relatively high doses of the substance in 17 of those examined.