Senior police officers have defended the use of undercover policing but said those officers had to remain within the law.
The comments from the Association of Chief Police Officers (Acpo) come in the wake of controversy surrounding Pc Mark Kennedy, who infiltrated a group of environmental activists.
The former Met officer monitored the actions of protesters across Europe in the guise of a climber called Mark Stone.
The case against six demonstrators accused of conspiring to shut down Ratcliffe-on-Soar power station collapsed on Monday after prosecutors dropped the case.
Their legal team claimed the decision was made after the undercover officer had a crisis of conscience and offered to give evidence on their behalf.
Independent officials are starting an investigation into whether police tried to cover up Mr Kennedy’s role amid claims in The Times that the trial collapsed when the CPS discovered Nottinghamshire Police withheld secret tapes of meetings.
The developments have sparked a wider debate on police tactics to monitor political and environmental groups operating on the fringe of legality.
A spokeswoman for Acpo said they could not comment on the case specifically but said they did not want to lose sight of the importance of undercover work.
She said: “The police service cannot operate effectively to prevent and detect crime unless it uses intelligence.
“It is one of the most challenging operational activities undertaken by the Police Service.”