TORY HQ will be breathing a sigh of relief this morning after the Crown Prosecution Service announced that no criminal charges will be brought against Conservative MPs or officials in relation to allegations of spending irregularities in the 2015 General Election campaign.

It insisted the claims brought were “politically motivated” and had wasted police time. It made clear that it believed all along that Tory candidates had done nothing wrong.

One file, relating to the victorious Conservative candidate in Thanet South, Craig Mackinlay, remains under consideration.

However, the CPS cautioned that this should not be taken as an indication of whether or not charges will be brought in this case, which was referred more recently than the others.

Nick Vamos, the CPS head of special crime, said that prosecutors considered files from 14 police forces but they determined that - while spending returns might have been inaccurate - there was insufficient evidence to prove that any candidate or agent had been 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 Mr Vamos.

Responding Patrick McLoughlin, the Conservative Party Chairman, said: “After a very thorough investigation, we are pleased that the legal authorities have confirmed what we believed was the case all along: that these Conservative candidates did nothing wrong.

“These were politically motivated and unfounded complaints that have wasted police time. We are glad that this matter is finally resolved.

“A number of false and malicious claims continue to be spread on the internet. People should be aware that making false claims about a candidate’s personal character and conduct is an electoral offence as well as being defamatory.”

He added: “Notwithstanding these false claims, Conservatives want to strengthen election rules to safeguard electoral integrity in light of the real and proven cases of electoral fraud exposed in Tower Hamlets in 2015.”

Allegations highlighted by Channel 4 News and the Daily Mirror related to busloads of Conservative activists sent to key seats, whose expenses were reported as part of national campaign spending rather than falling within the lower constituency limits.

In April, Scottish First Minister Nicola Sturgeon suggested that Prime Minister Theresa May had called a snap election before the probe "catches up with her".

Karl McCartney, the Conservative candidate for Lincoln, who was interviewed as part of the investigation, said heads should roll at the Electoral Commission watchdog, which conducted its own separate inquiries into the campaign.

"This whole saga amounts to no more than a politically-motivated witch-hunt," said Mr McCartney, who called for the immediate resignation of the Commission's senior management team.

"It is clear that those who lead the Electoral Commission who followed and allowed this action to take place are politically-motivated and biased - actions that have rendered this organisation wholly unfit-for-purpose," he said.

But Jeremy Corbyn, the Labour leader, said he was "surprised" by the CPS decision while Tim Farron, the Liberal Democrat leader, claimed the Tories had "driven a battlebus and horses" through the spirit of the law.

In March, the Conservative Party was fined a record £70,000 by the Electoral Commission for "numerous failures" in reporting its expenses for the 2015 General 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, West Yorkshire.

Mr Vamos said: "We considered whether candidates and election agents working in constituencies that were visited by the party's 'battle bus' may have committed a criminal offence by not declaring related expenditure on their local returns."

Under the 1983 Representation of the People Act, candidates and agents must sign declarations to state that their expenses returns are complete. To bring a charge it must be proved that they knew the return was inaccurate.

Mr Vamos said that in the 2015 cases, "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."

The CPS also considered whether there had been potential breaches of the technical offence of failing to deliver a true return. But Mr Vamos said that as agents had been told by Tory HQ that the costs of the bus visits were part of the national campaign, it would be impossible to prove any agent was knowingly acting dishonestly.

"We have concluded it is not in the public interest to charge anyone referred to us with this offence," he said.

Mr Vamos said that the CPS's evaluation of the evidence was "consistent with that of the Electoral Commission".