Foundation in memory of ex-spy

Marina Litvinenko set up the foundation in memory of her husband

The wife of murdered former KGB spy Alexander Litvinenko has said she will not rest until his killers are brought to justice at the launch of a campaign.

His widow, Marina, said she hopes the Litvinenko Justice Foundation will put pressure on the British and Russian governments to find those responsible.

Her husband died last November in London. His body contained high levels of radioactive substance polonium-210.

"It was not just one moment. I saw him dying for one month," she said.

Mrs Litvinenko was joined at the event in Whitehall by her husband's close friend Alex Goldfarb and Russian billionaire Boris Berezovsky.

She said: "I cannot say what I feel against the people who would do this because I do not know exactly who did it.

We want to bring everybody together to see whether we can help justice be done Alex Goldfarb

"I would like to know who did it. What I do is just for Sasha and his memory, for his son and for all people who should not expect this.

"I will not rest until Sasha's killers are brought to justice."

The main aims of the foundation are to maintain pressure on the UK and Russian governments to find Mr Litvinenko's killers, to prevent a repeat of the incident and to seek compensation for others who were exposed to the radioactive substance.

Mr Goldfarb told BBC Radio 4's Today programme: "There are hundreds of people who were exposed to polonium-210 in this terrorist attack in London.

"We want to bring everybody together to see whether we can help justice be done."

Compensation would be sought "as a group... from whoever will be found to be perpetrator of this crime", he added.

President blamed

Supporters of the foundation have accused the Russian government of poisoning Mr Litvinenko, an outspoken critic of President Vladimir Putin.

Russian Mr Litvinenko, 43, had defected to Britain before his death.

In a statement released after his death, he blamed the Russian president for his poisoning.

The Kremlin has always denied any suggestion that it was behind Mr Litvinenko's murder.

Until the method of this absolutely new type of killing involving a nuclear micro bomb is uncovered, not a single person in the UK or elsewhere in the world can feel safe against a similar murder Boris Berezovsky

Now the campaigners intend to press both governments for a conviction.

A letter from the foundation has been sent to Prime Minister Tony Blair, Home Secretary John Reid and Attorney General Lord Goldsmith asking for reassurance that suspects would stand trial in the UK.

And Mrs Litvinenko's lawyer, Louise Christian, is calling for a meeting with the Crown Prosecution Service (CPS), and said they may go to the European Courts of Justice and the United Nations.

She said: "We will not rest until the British and Russian governments have done their duty by Marina and of the victims of this terrible crime."

She called on the CPS to make public any evidence they have against suspects, even those not in Britain.

Mr Berezovsky, who was questioned for more than three-and-a-half hours last week by Russian officials, said more than half the questions had been about his finances.

He told those gathered for the event: "Alexander Litvinenko was my friend, a friend who saved my life."

He said the search for Mr Litvinenko's assassin was more than just a "personal crusade".

"Until the method of this absolutely new type of killing involving a nuclear micro bomb is uncovered, not a single person in the UK or elsewhere in the world can feel safe against a similar murder."

The Metropolitan Police said its investigation was still continuing and some information has been passed onto the Crown Prosecution Service.

The CPS said it would consider meeting Mrs Litvinenko and her lawyer.