The Leave.EU campaign breached electoral law over its Brexit referendum spending return, but will face no further police action.
Scotland Yard said there was “insufficient evidence” to meet the standard of proof required to launch court proceedings against the group led by Brexit Party MEP Nigel Farage.
“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,” police said.
Mr Farage said he was “very, very pleased to hear the news” the investigation had ended.
“I think the levels of accusation that have been thrown against these people from parliamentarians, even from the Electoral Commission, have been an absolute disgrace," he said.
The investigation began last May when the Electoral Commission referred Leave.EU to police over its 2016 spending and fined it £70,000.
It said the organisation had failed to include £77,380 in its spending returns, taking it 10 per cent over the limit for non-party registered groups.
The Commission said the amount of overspend may have been “considerably higher” as Leave.EU offered “incomplete and inaccurate” information.
An Electoral Commission spokesman has defended the decision to refer the case to police and said there were “reasonable grounds to suspect” the pro-Brexit campaign had broken the law.
A National Crime Agency investigation into Leave.EU founder Arron Banks over “suspected criminal offences” is ongoing. He has denied the allegations.
An investigation into the spending returns of Vote Leave and BeLeave is also ongoing.