Campaign group Leave.EU will face no further criminal investigation over its spending during the Brexit referendum.
It was referred to the Metropolitan Police after being fined £70,000 by the Electoral Commission in May last year for election law breaches.
But the Met said there was "insufficient evidence" to justify any further criminal investigation.
Leave.EU founder Arron Banks called for an inquiry to be launched into the commission's actions.
He said it had "serious questions to answer about political bias and whether it is fit for purpose as a regulator".
Mr Banks added that 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 National Crime Agency's investigation into the organisation is still ongoing, with a spokesman saying they will report its outcome "at the earliest opportunity, once all matters have been thoroughly investigated".
Scotland Yard said that - even though there had been "technical breaches" of electoral law by Leave.EU - it had taken the decision to drop its investigation after consulting the Crown Prosecution Service in early August.
Commander Alex Murray said: "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 May 2018 report, the Electoral Commission said Leave.EU had exceeded the spending limit for "non-party registered campaigners" by at least 10% by failing to include at least £77,380 in its spending return.
It also referred Ms Bilney to the police following its investigation.
An Electoral Commission spokesperson said: "We believed there were reasonable grounds to suspect that Ms Bilney had committed the offence of knowingly or recklessly making a false declaration about the Leave.EU spending return.
"This warranted thorough investigation and we therefore notified the police."
The spokesperson added: "As the Metropolitan Police note, this does not alter the findings of the commission's investigation from May last year, which found Ms Bilney to have committed four offences, including of submitting an inaccurate spending return and of exceeding the spending limit.
"Leave.EU appealed these findings, but the commission's position was upheld in court."