Fictional spies from James Bond to Austin Powers have long been famous for their raging, insatiable libidos, which are often directed at enemy agents.

Like most fiction, this has a basis in reality: Most famously, the dancer known as Mata Hari had a thing for military officers before she was shot by the French in 1917. Paris had accused her of being a German spy; the French considered her their agent.

More than 85 years later, the Canadian Security Intelligence Service and the U.S. Federal Bureau of Investigation are wrestling with love scandals that have shaken both agencies.

In Canada, news surfaced this week that a senior CSIS agent, Theresa Sullivan, was fired last year after falling in love with a source and carrying on a relationship for nearly two years. In Los Angeles, the FBI arrested Katrina Leung, a prominent businesswoman, this month; allegedly she slept with two FBI agents for 20 years and stole secrets from them for the Chinese government.

"Getting too close to the asset is the thing that most often brings an agent down," one senior U.S. law-enforcement official said. "It happens over and over again and it's usually about sex or money."

Intelligence agencies have always had problems controlling contact between agents and their sources, also known as "human assets." Even in this age of high-tech surveillance, human contact remains one of the most powerful intelligence tools. But human behaviour can't always be controlled and sometimes agents get too close for comfort.

"It's always a risk," said David Harris, former chief of strategic planning at CSIS. "It's a stress for many in intelligence who might find themselves developing some kind of relationship even of a non-romantic nature. It's just that sense of attachment that's normal to people."

Most services, including CSIS, have strict rules governing how agents deal with sources. Typically, officers have to submit written reports after every visit with a source and sometimes psychologists are called in to monitor an agent's emotional state.

But even that isn't enough in some cases. Ms. Sullivan was enjoying a stellar career until she fell in love with someone she was supposed to be spying on. She met a mysterious man -- who can only be identified to the public as "A.B." -- in 1996 while carrying out her duties for CSIS. When Ms. Sullivan's husband suddenly walked out on her in 1998, it was A.B. who sent her a sympathy card and a gold chain.

During long phone calls that followed, the professional relationship grew personal. The spy and her source dated, fell in love, even began to talk about marriage. Higher-ups told Ms. Sullivan that she was getting too close to a man considered a "person of interest." But she defied them.

When Ms. Sullivan and A.B. broke up in 1999, they did so only on their own terms. She had finally pulled herself together when Ward Elcock, the head of CSIS, sent a devastating letter to her.

"You consciously and willfully pursued a relationship with this individual after being instructed to cease contact," Mr. Elcock wrote in firing her. "It is with deep regret that I have taken this decision, but the gravity of your actions have left me no choice in this matter."

Today, Ms. Sullivan, 36, has given up ever being a spy again. But she feels that she has been judged too harshly. "We are talking about human beings and it's only human nature that things like that would happen," she said.

The Leung case is far more serious. Ms. Leung allegedly slept with two FBI agents over a 20-year period. During that time, the agents thought she was working for them and the FBI paid her a total of $1.7-million (U.S.). But according to allegations filed in court, Ms. Leung was really working for China. She has been arrested and is being held without bail.

Along with working for the FBI, Ms. Leung also managed a successful consulting firm that helped companies, including Canada's Nortel Corp., to develop business in China. Nortel paid her $1.1.-million (U.S.), which she put toward buying a Los Angeles mansion, and the company has insisted that it knew nothing about her undercover work.

One of Ms. Leung's lovers was her FBI "handler," James (J.J.) Smith. During meetings described as routine "debriefings" at Ms. Leung's lavish house, she is thought to have surreptitiously helped herself every now and then to the contents of Mr. Smith's briefcase.

Mr. Smith has been charged with gross negligence, but freed on $250,000 (U.S.) bail. He has denied any wrongdoing.

The other lover, retired agent William Cleveland, has not been charged. But in the wake of the scandal, he has had to quit his job -- he became the director of security at a prominent nuclear-research facility after leaving the FBI, and China has long been suspected of trying to get after U.S. nuclear-weapons secrets.

FBI director Robert Mueller called the arrest of Mr. Smith and Ms. Leung "a sad day for the FBI" and he promised changes to the agency's practices and procedures.

But Ms. Leung's family recently released a Chinese-language statement saying the FBI has been "trying to protect itself and shift the blame" since learning that its own agents were "very sloppy."

"The FBI is doing what they have done in other cases of FBI bungling," the statement said. "Their people in Washington are orchestrating things to protect their own. They blame the non-agent and the foreign born, especially the Asian, especially the woman.

"When the FBI is embarrassed, they revert to their old ways. They use a double standard to blame outsiders and protect their own."

The Leung affair isn't the first time an FBI agent has been caught in a compromising position. In fact, when sex and spies mix, the results can be tragic.

In 1989, FBI agent Mark Putnam killed informant Susan Daniels Smith after she threatened to expose their affair -- and her pregnancy. After strangling her, he drove her body around in his trunk for one day, before dumping it in a ravine. He was recently paroled after serving 10 years of his 16-year prison sentence.

In 1984, an L.A. counterintelligence agent named Richard W. Miller was convicted of trading secrets for sex with a female KGB agent, but there was also a promise he would be paid $65,000 (U.S.) in cash and gold. He was sentenced to 20 years in prison.

Canada's spies are not immune to sex scandals, but so far Ms. Sullivan's case is one of the few to become public.

She says agents have been privately told about other cases for years. "In training, these are things that you are forewarned about," she said. "There were examples given without names being mentioned. I am aware that it has happened in the past."

She laughed when asked if CSIS believes female agents are more prone to affairs than male agents. "I would say that maybe that's what their understanding is. I would say there are male and female employees that are vulnerable on different levels."

Michel Juneau-Katsuya, a veteran CSIS agent turned security consultant, said he has no doubt that CSIS spies are just as vulnerable as FBI agents to falling into illicit affairs.

In 1997, he co-wrote a famous report known as Sidewinder, which alleged that Chinese spies were working closely with criminal gangs in Canada. Ultimately, however, the report's findings weren't supported by his bosses. "CSIS said this is a conspiracy theory type of thing and totally discarded the report," Mr. Juneau-Katsuya said. "In Canada, we still have denial that the Chinese are doing such things."

He said he doesn't know of any Chinese spies sleeping with Canadian agents. But sex, he said, is one of the oldest tricks in the spy-agency books. The Soviets once established schools to train comely agents in the art of seduction, he said, and CSIS takes steps to control its agents' potentially perilous pillow talk.

"We remind the recruits they shouldn't be doing those things," Mr. Juneau-Katsuya said. "If it's a male controller meeting with a female source, we usually send two persons. . . . Because most of the meetings take place in private places like a hotel. It's very easy to let things go overboard."

The lives of CSIS agents, however, don't resemble that of James Bond: Lavish salaries, tuxedos, Bond girls and five-star hotels are in short supply. So agents have been known to take their perks where they can get them, leading to the occasional misuse of rented rooms.

"After they met their source, they met their girlfriend in the hotel room and stayed for the night," Mr. Juneau-Katsuya said. "The logic they're using is the room is paid for anyways."

For such indiscretions, he said CSIS has had to reprimand its employees.

Reid Morden, former director-general of CSIS, said tales of sexual misconduct by agents are rare, but controlling human assets is not easy.

"Human assets are very volatile species," Mr. Morden said. "They are doing work which could be quite difficult if not dangerous and so both their mental and emotional temperature has to be taken very frequently. And, consequently, people work very hard to see that it is kept on a very professional basis. But every once in a while the human factor gets in the way."

Mr. Morden said that managing sources is becoming even more critical in the aftermath of the Sept. 11, 2001, terrorist attacks. Many agencies, including CSIS, have discovered that basic human contact is the best way to develop effective countermeasures to terrorism, he noted.

"A number of us would say that one of the reasons that there was this massive intelligence failure on Sept. 11 was that the Americans had become so bedazzled by the amount and strength of the technology that they had that they put a lot of resources into electronic intelligence and other kinds of mechanical things and maybe forgot the importance of the human source," he said.

But that extra contact, he added, will mean agencies will have to be more on guard to avoid improper relations between agents and sources. "Everybody is human," he said. "If they were all machines, it would never happen."

Paul Waldie is a Globe and Mail feature writer and Colin Freeze is The Globe's crime reporter.