Convicted spy Igor Sutyagin had been facing serving out his sentence in hard regime prison in Russia's Far North, but now is at the centre of a mooted swap of prisoners with the United States.

Sutyagin, a Russian arms control expert, 45, was convicted in 2004 of handing over classified information to a British company that Russia claimed was a CIA cover, and sentenced to 15 years in jail.

The civilian academic with degrees in physics and history was accused of handing over sensitive data on nuclear submarines and missile warning systems to the British consulting firm Alternative Futures, where he worked as a freelancer.

He pleaded not guilty, saying the information came from open sources and was not a state secret.

His lawyer and relatives said Wednesday he was informed he would be freed and sent to Britain as part of a swap of detainees with the United States to end the spy scandal sparked by the arrest of 10 suspected Russian agents.

They said his choice to sign a document admitting his guilt in exchange for a deal to be released and leave Russia was a hard decision as he had always insisted on his innocence.

His brother Dmitry said, "It was the first time I saw him so low. He looks as if he has lost a lot of weight. For him, it was all a huge shock, something completely unexpected."

"It was an offer that was impossible to refuse," Dmitry Sutyagin said.

Sutyagin was first detained in October 1999 in his home town of Obninsk in the Kaluga region, about 100 kilometres (60 miles) from Moscow, and questioned by security services.

He was charged in November that year with treason, facing up to 20 years in jail, but in a long and convoluted process, he was not convicted until 2004, as the first trial broke down.

Human rights activists appealed to the FSB security service to free him, saying he had no access to secrets and openly worked with foreign academics.

He had been serving his sentence in a hard regime prison in the region of Arkhangelsk in the remote and freezing Far North of Russia.

"There is very little that seems to suggest any wrongdoing, let alone the very serious crime of high treason," human rights activist Lyudmila Alexeyeva wrote to the director of the FSB security service, Nikolai Patrushev, in July 2000.

Finally, in April 2004 he was found guilty by a jury trial. His sentence of 15 years was the longest for espionage since Soviet times. Media watchdog Freedom House in a 2005 report described his trial as "politicized."

In 2004, Amnesty International expressed deep regret at his conviction, saying it was concerned that the case was politically motivated and he was being denied the right to a fair trial.

He appealed to the European Court of Human Rights and his lawyer said this spring that his case was expected to be heard in 2010.

In 2007, he asked then-president Vladimir Putin for a pardon, but it was not granted, because he did not admit his guilt. In May this year, he was denied parole for disciplinary offences including not clearing crumbs from his table.

Until his detention, Sutyagin apparently led the retiring life of an academic.

Married with two daughters, he worked as a researcher at the respected USA/Canada Institute in Moscow, specialising in arms control and was sometimes quoted as an expert by journalists.

He conducted research with US and Canadian universities and travelled to conferences.

He grew up in Obninsk, a centre for nuclear research, where his parents worked at a scientific research institute.

According to a biography on a web site set up by his family, he headed the Young Pioneer and Komsomol Communist groups at his school, which he graduated with top grades in all subjects.