A disciplinary hearing found Det Sgt Jan Beasant guilty of misconduct following a review of the investigation into the murder of Blackpool teenager Charlene Downes.
Iyad Albattikhi was cleared in 2008 of killing her after "grave doubts" were raised around the evidence.
Lancashire Police said Ms Beasant had shown conduct that "let everyone down".
Two other officers who retired prior to the Independent Police Complaints Commission (IPCC) investigation could not be considered for disciplinary sanctions.
Charlene Downes, 14, disappeared in 2003 and has not been seen since. Mr Albattikhi was arrested for her alleged murder in 2007, following covert surveillance.
A jury at Preston Crown Court was discharged in 2007 after failing to reach a verdict and a subsequent retrial collapsed after the Crown Prosecution Service conceded it had "grave doubts" about the reliability of the covert surveillance.
Ms Beasant had the job of transcribing secretly recorded conversations between Mr Ilbattikhi and another man, spending 2,500 hours over two years listening to 52 audio tapes.
The quality of the covert recordings was criticised during the trial by defence barristers as "poor" with confidence "low" in the accuracy of the transcriptions.
A review by the IPCC concluded the investigating team were guilty of a strategic and tactical failure in the management of the material.
Naseem Malik, IPCC Commissioner for the North West, said it was "abundantly clear" that the covert surveillance was "handled poorly and unprofessionally".
The watchdog recommended Ms Beasant face a disciplinary hearing, one officer should receive a written warning and five others should receive words of advice.
At a hearing earlier this week, Lancashire Police found her guilty of two counts of misconduct and forced her to resign.
Supt Simon Giles said the force "expects the highest professional standards from all our staff and the panel has found this individual's conduct has fallen well short of these standards".
"This sort of behaviour and conduct lets everybody down - not just the police service but those the police serve," he said.
"It is appropriate they have faced the consequences of their actions."
A Lancashire detective has been forced to resign after an investigation into the handling of a murder case.