Alexander Litvinenko was being paid about $4,000 a month by MI6, The Daily Mail reported in an exclusive published Saturday.
Litvinenko, 43, who died Nov. 23, 2006, was recruited as an MI6 agent by Sir John Scarlett, who now heads the intelligence service and previously was based in Moscow, the Mail reported.
Andrei Lugovoy, believed by British agents to have poisoned Litvinenko, has said Litvinenko tried to recruit him to supply information to MI6 when the two met Nov. 1, 2006 at the Millennium Hotel in London.
Lugovoy, a businessman, and a former KGB agent himself, has denied killing Litvinenko and refuses to return to Britain for prosecution. The Kremlin has refused Britain's extradition request, causing a diplomatic chill between the two countries.
British authorities believe radioactive polonium 210 was slipped into a pot of tea when Litvinenko and Lugovoy met at the hotel.