No Conservatives will face charges for breaches of expenses rules over the 2015 general election "battle bus".
The Crown Prosecution Service said it had examined files from 14 police forces and while there was evidence of inaccurate spending returns it did not "meet the test" for further action.
It is still investigating the Conservative campaign in South Thanet.
The Conservatives said the allegations had been "politically motivated" and a "waste of police time".
The inquiry related to claims some "battle bus" campaigning costs in the 2015 election were wrongly recorded.
It was alleged the party spent tens of thousands of pounds on local campaigns - including on "battle bus" visits by activists - which were either not declared or were wrongly registered as national spending.
Under complex election spending rules, parties must disclose national spending separately from money used to promote each candidate, and separate national and local limits apply.
In March the Tories were fined a record £70,000 by the Electoral Commission, which criticised the party for resisting its investigation although it was unable to prove any intention to make false declarations.
But following a lengthy investigation, the Crown Prosecution Service said it would take no further action against either candidates or party officials.
It said a file from Kent Police on the South Thanet campaign was recently received and was "under consideration", adding that nothing should be inferred from the fact that its inquiries were continuing.
A decision on whether to bring charges is likely to be made before the General Election, BBC home affairs correspondent Danny Shaw understands.
In a statement, the CPS said it was an offence to knowingly make a false declaration but in order to bring charges it must be proved that suspects knew the return was inaccurate and acted dishonestly in signing the declaration.
"Although there is evidence to suggest the returns may have been inaccurate, there is insufficient evidence to prove to the criminal standard that any candidate or agent was dishonest," said its head of special crime Nick Vamos.
Submitting inaccurate returns relating to battle bus spending was a "technical offence", Mr Vamos said, but he did not believe in the circumstances it was in the public interest to bring charges against individuals.
Speaking at an event in Nottingham, Prime Minister Theresa May said the Tories had believed all along local expenses had been properly reported and declared.
All the major parties had been fined for mistakes on national expenses, she said. "We've paid our fines, and I sincerely hope the other parties are paying theirs."
Conservative Party chairman Patrick McLoughlin said the party regretted the police had become involved.
"These were politically motivated and unfounded complaints that have wasted police time," he said.
The Electoral Commission said the CPS's findings were consistent with its own investigation, which had found the Conservative's spending return was incomplete and inaccurate.
The allegations came to light following an investigation by Channel 4 News and the Daily Mirror.
Labour leader Jeremy Corbyn said he was "surprised" by the ruling and would "look into the details". While he respected the independence of the Crown Prosecution Service, he said the electoral laws had to be adhered to and enforced.
Lib Dem leader Tim Farron said the Tories had "driven a battle bus and horses" through the spirit of the law.
One Conservative MP investigated by the police, Karl McCartney, called on senior officials at the Electoral Commission to resign.
Describing the watchdog as "unfit for purpose", Mr McCartney - who is seeking re-election in Lincoln - said unless senior figures took responsibility he would campaign for the quango's abolition after the general election.