It comes after Jermaine Baker, 28, was killed by armed officers in London, in December 2015.
Mr Baker, who was with two men in a car near Wood Green Crown Court, had intended to help a man being held on remand escape from a prison van.
Firearms officers were deployed to disrupt the plan.
"The officers reasonably believed that the men in the car were dangerous individuals, who were armed and prepared to use their weapons to achieve their criminal purpose," a Crown Prosecution Service (CPS) spokesman said.
"Having taken account of all the evidence provided by the IPCC [Independent Police Complaints Commission], the CPS has concluded that there is not a realistic prospect of conviction."
"The prosecution could not prove to the required standard that [the officer] was being untruthful about his belief that Mr Baker was armed and reaching for a weapon to fire on the officers."
The CPS spokesman said the officer had fired one shot which hit and killed Mr Baker, from Tottenham.
"Following a thorough search it was found that Mr Baker was not armed," he continued.
"An imitation firearm was, however, recovered from a holdall in the rear foot well behind the driver's seat of the car."
The IPCC said it was aware of the CPS decision and was waiting from a response from the Met Police.
At the time, Mr Baker and his accomplices were attempted to spring from a prison van Izzet Eren, who was being held on remand at HMP Wormwood Scrubs accused of gun offences, as he was en-route to a hearing at the court.
During the subsequent trial of one of Mr Baker's accomplices, Eren Hasyer, jurors at Woolwich Crown Court were told the prisoner hatched the escape plot from his cell using a smuggled mobile phone.
Det Ch Supt Tom Manson, of the Met Police, said: "This was a bold, well planned and carefully thought out conspiracy that bears all the hallmarks of a professional crime."
Two other men, Nathan Mason and Gokay Sogucakli, had admitted being part of the escape plot before Hasyer's trial began.
Hasyer was found guilty in May last year, of aiding the attempt to free Eren from the prison van.
A Met Police firearms officer will face no charges over the shooting of a man who was killed in a botched attempt to free a prisoner, prosecutors said.