 When is a lie morally justifiable? By coincidence, two of Britain's leading public watchdogs have just set themselves the task of answering that question, though in rather different circumstances. and their answer might be: not when sex is involved.  The bizarre case of the undercover police officer who spent seven years posing as an eco-warrior, before seeing the light on climate change and switching sides, is to be investigated by the Independent Police Complaints Commission. among the issues it will address are whether PC Mark Kennedy crossed the line from covert surveillance to become an agent provocateur. He also had sex with a series of women activists who fell for the crustie copper with his long hair, eco-stubble and exotic tattoos. He cut a romantic figure with his eagerness for clambering up trees and protest cranes - and plenty of what turned out to be taxpayers' money to splash around in the cause of the climate.  On the same day, the Press Complaints Commission (PCC) announced that, after 200 letters from the public, it is to launch an inquiry into the Daily Telegraph's subterfuge last month of sending two glamorous reporters, posing as young mums with benefits problems, to pretend to be constituents of various Lib Dem ministers - and coax a variety of tape-recorded indiscretions from them. The most high-profile casualty of the sting was the Business Secretary, Vince Cable, who imprudently boasted that he had "declared war" on Rupert Murdoch.  Telling lies to get to the truth has a chequered history among both police and press in modern Britain. Which is why the PCC has a code of practice which declares that "clandestine devices and subterfuge" can be justified "only in the public interest and when the material cannot be obtained by other means". The problem is that this merely shifts the problem to a different level.  Some subterfuge is perfectly proper. The police undercover unit to which PC Kennedy belonged began life in the late Nineties to infiltrate animal rights militants who were sending letter bombs to scientists, intimidating researchers at home and, most notoriously, digging up a grandmother's grave. Few would question the ethics of the then National Public Order Intelligence Unit penetrating such a cell by deception.  But the single greatest impulse of any institution is self-perpetuation. When the threat of animal rights extremism had been vanquished, those who ran such units looked for new targets to justify their existence. They got their brief expanded to cover all "domestic extremists", from far-right racists to deep-green eco-activists.  Virtually any protester became fair game, including activists who wanted to shut down a coal-fired power station for a few days as a global warming protest. That was a serious threat, said Sir Hugh Orde, president of the association of Chief Police Officers, this week, since the electricity supply for two million people, including hospitals and a lot of vulnerable people, could be affected. But it is hard to put that in the same category as those who want to kill scientists or blow up Tube trains.  Of course, climate change actions can be hijacked by violent individuals whose primary motivation is to seek street conflict with the police. But the vast majority of climate campaigners are, in the words of a judge in a related case, "honest, sincere, conscientious, intelligent, committed, dedicated and caring".  So a key question is whether the response is proportionate. Equivocations, concealments, exaggeration, half-truths, disguises and downright lies can be justifiable, but who decides, and how. To judge from the volume of complaints about the Telegraph's covert recording of government ministers, of one party only, the public sees countervailing ills from journalistic stings. Words such as dishonest, low, deceitful and cheating were used by critics. Reporters had done great damage to the confidentiality of the relationship between MPs and constituents, which was akin to that between doctors and patients or lawyers and clients.  That is a fair point, even if what the MPs said smacked of the silly boasting of middle-aged men anxious to impress young women, one of them described as "really quite attractive".  The sexuality was less latent in the case of PC Kennedy. In the course of his eco-activism he had relationships with a number of women who believed him to be a fellow activist. Whether he was sleeping with them to extract information or because he was seduced by their cause is unclear. But Kennedy did not merely watch from the back of the bus. Rather, he drove it, ferrying people and goods to protests, hiring vans and paying the court fines of his fellow activists.  What was his motivation? Was he diverting protesters into activities that police regarded as less dangerous, as when he proposed targeting buses of a racist group rather than confronting them in a mass disorder in Bradford city centre? Or was he cleverly undermining act 