The Leave.EU campaign will not be subject to any further action due to technical infractions to the electoral law in connection with the return of its referendum expenses, police said.
Scotland Yard said there was "not enough evidence" to warrant further investigation on the pro-Brexit group, founded by businessman Arron Banks and led by the former head of the UKIP, Nigel Farage.
The force made the decision after receiving notice from the Crown Prosecution Service following a referral on August 5.
Leave.EU was fined £ 70,000 by the Electoral Commission last May for offenses committed under the electoral law.
It followed a regulator's survey on campaign group financing and spending in the 2016 EU referendum.
The Electoral Commission, which referred the case to the police, said the group had incorrectly declared what it had spent for the referendum.
This included not including at least £ 77,380 in the return of his expenses, which meant that the group exceeded by at least 10% the spending limit for registered non-political party activists.
The Metropolitan Police said in a statement released on Friday: "It is clear that, if technical violations to the electoral law were committed by Leave.EU regarding the return of the expenses submitted for their campaign, the evidence is insufficient to justify a subsequent criminal investigation.
"The person in charge of Leave.EU has been informed that she would not face any action from the police."
Commander Alex Murray, Crimes Specialist at the station, said: "It was just to investigate the allegation, but after extensive investigations, it became apparent that the nature of the potential offenses at the plant, the regulation, the criminal standard of proof required by the court and the measures taken by LetE.U adhere to the regulation means that it is now appropriate to do nothing. "
Banks, president of Leave.EU, called for the resignation of the president of the electoral commission.
He said: "The shameful political collusion between the Electoral Commission and Damian Collins' DCMS (Digital, Culture, Media and Sport) Committee, and a number of prominent parliamentarians, testifies to a serious abuse of office. will require a full public inquiry into their actions.
"We will write to the Prime Minister today to request a public inquiry into the actions of the Electoral Commission.
"I request the resignation of Damian Collins and the CEO of the Electoral Commission following the news today.
"The Electoral Commission has serious questions about political bias and its relevance as a regulator."
He added that the probe had had a "heavy personal toll" on Leave.Eliz's manager, Liz Bilney, and her family, and accused MPs of "anti-Brexit" of "harassment and lies".
The force said that the Vote Leave and BeLeave expense return surveys were ongoing.