Federal Security Service official Viktor Komogorov said Sunday his country was fully prepared to aid British investigators in their search for additional information about the November 2006 assassination of the former Russian security service officer, RIA Novosti said.

"We are ready for cooperation and interaction with them (British security services) but the first step should be made by Britain. We expect them to apologize for unfounded accusations as we are absolutely uninvolved in what they accuse us," Komogorov said.

The investigation into the apparent radioactive poisoning case has caused relations between the two countries to deteriorate dramatically, RIA Novosti said.

The Russian press agency said this was due in part to Russia's refusal of a British extradition request for Russian businessman Andrei Lugovoi, Britain's top suspect in the London assassination plot.