LONDON — Alexander Litvinenko, the former Russian agent-turned-Kremlin critic, was a ‘‘registered and paid’’ agent working for Britain’s foreign intelligence agency when he died after being mysteriously poisoned, a lawyer representing his widow told an official hearing Thursday. Another lawyer said the UK has evidence the Russian government was behind Litvinenko’s death.

The 43-year-old Russian died in November 2006 after drinking tea laced with the rare radioactive isotope polonium-210 at a London hotel. Six years later, British authorities are reopening investigations into the shocking circumstances of his death.