This sounds like a movie or detective tale. But The Guardian reports that it's for real:
"A police spy married an activist he met while undercover in the environmental protest movement and then went on to have children with her, The Guardian can reveal.
"He is the fourth spy now to have been identified as an undercover police officer engaged in the covert surveillance of eco-activists. Three of those spies are accused of having had sexual relationships with the people they were targeting."
The undercover cops were members of London's Metropolitan Police force. Department officials tell the Guardian they are investigating, and that it's "never acceptable" for undercover officers to engage in such relations. Copyright 2011 National Public Radio. To see more, visit http://www.npr.org/.