No further action will be taken against Brexit campaign group Leave.EU over "technical breaches" of electoral law, Scotland Yard said.
The Met Police said that there was "insufficient evidence" to justify any further criminal inquiries into spending by the group, founded by businessman Arron Banks and spearheaded by Nigel Farage, during the referendum in 2016.
A Met statement said: "On 5 August 2019 the MPS submitted a file to the CPS for Early Investigative Advice in relation to the Leave.EU investigation and this advice has now been received.
"It is clear that whilst some technical breaches of electoral law were committed by Leave.EU in respect of the spending return submitted for their campaign, there is insufficient evidence to justify any further criminal investigation.
"Leave.EU's responsible person has been has been told that they will face no further police action."
Both Mr Banks and Mr Farage - who now heads the Brexit Party - had always denied any suggestion of wrongdoing
The force said investigations into the spending returns of Vote Leave and BeLeave were ongoing.
Commander Alex Murray, of Central Specialist Crime, said: "It was right to investigate the allegation, however following detailed enquiries it became apparent that the nature of potential breaches of the regulations, the criminal standard of proof required in court and the actions taken by Leave.EU to adhere to the regulations, mean that it is now appropriate to take no further action."
In a statement, Mr Banks called for the resignation of the head of the Electoral Commission and Damian Collins MP, who sits on the Digital, Culture, Media and Sport committee for their role in pressing for an investigation.
"We will be writing to the Prime Minster today demanding a public inquiry into the actions of the Electoral Commission," he said.