The widow of poisoned Russian spy Alexander Litvinenko has vowed to continue her quest for the truth after the Government refused her calls for a public inquiry into his death.

Marina Litvinenko said she was still "optimistic" that the full circumstances behind her husband's death would be exposed.

Her comments came after suffering another devastating setback in her pursuit of justice, as coroner Sir Robert Owen said the Government had rejected calls for a public inquiry to be set up in place of an inquest.

Outside the Royal Courts of Justice, in London, Mrs Litvinenko said: "Everyone was shocked and disappointed by the decision."

She added she wants to know why the Government made its decision.

Mrs Litvinenko said: "Were they trying to protect the Russian state? Were they trying to protect national security secrets?"

Mrs Litvinenko continued: "I'm optimistic, now so many people are involved in this case. More and more people want to know the truth."

She added that the way the Government handled its decision not to hold a public inquiry was "impolite and rude".

Alex Goldfarb, a friend of the Litvinenko family, attended the hearing to show his support.

Mr Goldfarb said: "It's absolutely transparent that the Russian government is behind this murder. The evidence has been seen by the Coroner and the courts.

"There's prima facie evidence that the Russian government is behind it.

"There's some sort of collusion with Her Majesty's Government and the Kremlin to obstruct justice."

Elena Tsirlina, Mrs Litvinenko's solicitor, said the decision not to hold a public inquiry followed "months of talks between the two governments at the highest level" between the prime ministers of both Russia and Britain.

Mr Litvinenko, 43, was poisoned with radioactive polonium-210 while drinking tea at the Millennium Hotel in London's Grosvenor Square in 2006. The family believe he was working for MI6 at the time of his death.