The Leave.EU campaign will face no further action over technical breaches of electoral law in connection with its referendum spending return, police said.
Scotland Yard said there was "insufficient evidence" to justify further investigation into the pro-Brexit group, founded by businessman Arron Banks and spearheaded by former UKIP leader Nigel Farage.
The force made the decision after receiving advice from the Crown Prosecution Service following a referral on 5 August.
Leave.EU was fined £70,000 by the Electoral Commission in May last year for offences committed under electoral law.
It followed an investigation by the regulator into the campaign group's funding and spend during the 2016 EU referendum.
The Electoral Commission, which referred the case to police, said the group incorrectly reported what it spent on the referendum.
This included failing to include at least £77,380 in its spending return, meaning the group exceeded the spending limit for non-party registered campaigners by at least 10%.
The Metropolitan Police said in a statement on Friday: "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."
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."
Mr Banks, Leave.EU's chairman, called for the resignation of the head of the Electoral Commission.
He said: "The disgraceful political collusion between the Electoral Commission and the Damian Collins 'Remain-biased' Digital, Culture, Media and Sport (DCMS) committee, and a number of leading Remain MPs, demonstrates a serious abuse of public office and we will be demanding a full public inquiry investigation into their actions.
"We will be writing to the prime minister today demanding a public enquiry into the actions of the Electoral Commission.
"I am calling for the resignation of Damian Collins and the CEO of Electoral Commission as a result of today's news.
"The Electoral Commission have serious questions to answer about political bias and whether it is fit for purpose as a regulator."
He said the probe had taken a "huge personal toll" on Leave.EU chief executive Liz Bilney and her family, and accused "anti-Brexit" MPs of "harassment and lies"
The force said investigations into the spending returns of Vote Leave and BeLeave were ongoing.