The Crown Prosecution Service said it had considered files from 14 police forces, but determined that while spending returns may have been inaccurate, there was insufficient evidence to prove any candidate or agent was dishonest.
“We reviewed the files in accordance with the Code for Crown Prosecutors and have concluded the tests in the Code are not met and no criminal charges have been authorised,” said head of special crime Nick Vamos.
One file, relating to the winning Conservative candidate in Thanet South, Craig Mackinlay, remains under consideration.
But the CPS said this should not be taken as an indication of whether charges will be brought in this case, which was referred more recently than the others.
The announcement came just a day before the deadline for nominations for the election next month, and lifted a shadow which some feared might hang over the party’s campaign.
The police investigations centred on allegations that expenses relating to busloads of Conservative activists sent to key seats were reported as part of national spending rather than within the lower constituency limits.
The Conservative Party was fined a record £70,000 by the Electoral Commission in March for “numerous failures” in reporting its expenses for the 2015 election and three by-elections in 2014.
The identities of MPs and officials under investigation have not all been revealed.
But the CPS received files of evidence from police forces in Avon & Somerset, Cumbria, Derbyshire, Devon & Cornwall, Gloucestershire, Greater Manchester, Kent, Lincolnshire, Metropolitan, Northamptonshire, Nottinghamshire, Staffordshire, West Mercia, West Midlands and West Yorkshire.
Following the CPS decision, Theresa May said: “We have always reported expenses according to the rules. What the CPS found very clearly in those cases they looked at in local constituencies was that local spending had been properly recorded – the candidates had done nothing wrong.
“There was an error made in our national return for the 2015 General Election. The Electoral Commission fined us for that, and we have paid that fine. Other parties were also fined for errors that were made in their 2015 election expenses.”
Conservative Party chairman Sir Patrick McLoughlin said the claims were “politically motivated and unfounded complaints that have wasted police time”.
The Conservative candidate for Lincoln, Karl McCartney, who was interviewed as part of the investigation, called for heads to roll at the Electoral Commission, which conducted its own separate inquiries.
“This whole saga amounts to no more than a politically motivated witch-hunt,” he said.
Labour leader Jeremy Corbyn said he was “surprised” by the decision, while Lib Dem leader Tim Farron said the Tories had “driven a battle bus and horses” through the spirit of the law.
An Electoral Commission spokesman said the CPS’ evaluation was “consistent with that of the Commission, which concluded that the Conservative party’s spending return was incomplete and inaccurate, as it contained spending that should have been included in the candidates’ returns”.
Under the 1983 Representation of the People Act, candidates and agents must sign declarations stating their expenses returns are complete.
To bring a charge it must be proved they knew the return was inaccurate.
As well as deciding there was insufficient evidence to prove any candidate or agent was dishonest, prosecutors also concluded it was not in the public interest to pursue charges on the technical offence of failing to deliver a true return.
As agents were told by Tory HQ that the costs of the bus visits were part of the national campaign, it would be impossible to prove that any of them had knowingly acted dishonestly.