The start of a long-delayed inquest into the death of a former Russian intelligence agent poisoned in London has been postponed for another five months.

Alexander Litvinenko - an ex agent who had turned Kremlin critic - died in November 2006 after drinking tea laced with the radioactive isotope polonium-210.

Lawyers for his family say that at the time of his death he was working for Britain's intelligence services.

Britain accuses two Russians of the killing, but Moscow refuses to extradite them.

The inquest has been preceded by legal wrangling over whether some evidence should be heard in secret for security reasons.

Coroner Robert Owen said Thursday that he was postponing the inquest until Oct. 2 "with great reluctance" because delays in disclosing evidence made the planned May start unrealistic.