WASHINGTON Former FBI agent Robert Hanssen pleaded guilty Friday to charges of spying for Moscow and will spend the rest of his life in prison under a plea-bargain that spares him the death penalty, AFP reported.

In return, Hanssen, who pleaded guilty to 15 counts of espionage and conspiracy, will fully cooperate with investigators probing his spying career, his lawyer Plato Cacheris said.

The 57-year-old senior FBI counterintelligence officer was charged with spying for Moscow from 1985 in return for some 1.4 million dollars paid in cash and diamonds.

Cacheris revealed, however, that Hanssen had begun spying even earlier -- in 1979 and stopped twice, before resuming espionage for Russia in 1999.

He said his client "very much wanted to make amends. That's a big reason for this disposition today and he wanted to tell his former agency what he had done and why he had done it."

Deputy U.S. Attorney General Larry Thompson said the government was reluctant to forgo the death penalty. But the plea bargain was the only way to "fully access the magnitude and scope of Hanssen's espionage activities," he said.

The plea agreement will also avert a jury trial scheduled for October, to the relief of U.S. spy agencies troubled by a public airing of secret information. Instead Hanssen will be sentenced in January to a life term in prison.

Cacheris, who represented CIA veteran Aldrich Ames who was jailed for life in 1994 after a lengthy spying career for Moscow, said that the government will learn "invaluable" information from Hanssen.

"They are going to learn things that they did not know (including that) his activities commenced in 1979 to 81 and that period the government knew nothing about," he said.

Hanssen had virtually unlimited access to state secrets and is believed to have seriously compromised U.S. national security. He also gave the KGB information on three Russian double agents, two of whom were executed.

Cacheris said that the damage done by Hanssen was "about the same as Aldrich Ames," who spied for the KGB for nearly a decade, passing thousands of classified documents to Moscow.

Hanssen was arrested February 18 in a sting operation after dropping a batch of documents at a location agreed by his Russian handlers in a public park in Virginia. Cacheris said the father-of-six had a premonition he would be found out that day.

Following the arrest, Washington ordered home 50 Russian diplomats, four of them linked to Hanssen, and Moscow replied with a tit-for-tat expulsion order in the worst post-Cold War spying spat with Russia.

Under the plea agreement, Hanssen was ordered to forfeit the income from spying although his wife Bernadette, who has fully cooperated with the investigation, will keep their house and three cars and receive part of the 27-year FBI veteran's pension.

Investigators said Hanssen, a devout Roman Catholic, was a meticulous double agent so secretive he never met his Russian handlers and they never knew his real identity.

"He never met any Russians. They did not know him," Cacheris said. "I think he was pretty good."

Cacheris said the FBI will benefit from learning "how he did it, how he got away with it."

Debriefing sessions are scheduled to take place over the next several months. If investigators feel that Hanssen is withholding information, the plea bargain could be rescinded and the case go to trial with the former agent again facing the death penalty.