The solicitor for six green campaigners says police need to answer "serious questions" about an undercover officer who infiltrated their group.
Mike Schwarz said efforts to charge the six with conspiring to shut down the Ratcliffe-on-Soar power station in Nottingham were a "serious attack on peaceful, accountable protest".
The case was suddenly dropped.
It emerged that undercover Pc Mark Kennedy had offered to give evidence on the campaigners' behalf.
Mr Kennedy had been intimately involved in the green movement since 2000. He told the defence team he would be prepared to help them, and the prosecution then offered no evidence before the trial was due to get under way.