TORY high command has lashed out at “politically motivated” allegations of election expenses fraud made against the party after prosecutors said no criminal charges would be brought regarding up to 20 Conservative candidates.

Nevertheless, Nicola Sturgeon insisted the Tories still had "significant questions to answer" over their election expenses and called for a review of the Electoral Commission's "clout".

Jeremy Corbyn, the Labour leader, said he was "surprised" by the Crown Prosecution Service’s decision, and Tim Farron, the Liberal Democrat leader, said, that while Tories might have observed the letter of the law, they had "driven a battlebus and horses" through its spirit.

"The Conservative Party have very clearly broken the spirit of the law, very clearly used the ability for them to spend the money that they have in order to affect the outcome of the last election," said Mr Farron during an election visit in south west England.

Police investigations into at least 30 individuals had centred on allegations highlighted by Channel 4 News and the Daily Mirror that expenses relating to busloads of Conservative activists sent to key seats were reported as part of national spending rather than falling within the lower constituency limits.

The decision by the CPS not to proceed would have been met with relief at Tory HQ as it came just a day before the deadline for nominations for the June 8 General Election. It lifted a shadow, which some feared might have hung over the party’s entire campaign.

The prosecuting authority explained that, even though spending returns submitted by some of the Conservative candidates and officials might have been inaccurate, there was insufficient evidence to prove they were knowingly dishonest.

One file, relating to Craig Mackinlay, who saw off the challenge of Ukip’s Nigel Farage in South Thanet in Kent, remains under consideration. But the CPS cautioned that nothing should be read into this fact; the file was submitted later than the others.

Theresa May, on the stump in Nottinghamshire, said the prosecutors’ decision had confirmed Tory expense returns were "properly reported and properly declared" and that the party’s election candidates under investigation had done nothing wrong.

The Prime Minister said those who had promoted the allegations should "consider the basis on which they made those complaints", which had taken up police time.

One of those investigated, Karl McCartney, the Conservative candidate in Lincoln, called for heads to roll at the Commission, which he accused of a "politically-motivated witch-hunt".

Sir Patrick McLoughlin, the party Chairman, was also critical, saying: "These were politically motivated and unfounded complaints that have wasted police time. We are glad that this matter is finally resolved."

Tory HQ has always insisted that any discrepancies in the figures were simply down to administrative errors.

In March, the party was fined a record £70,000 by the Commission for "numerous failures" in reporting its expenses for the 2015 General Election and three by-elections in 2014.

"If you look at the expenses issue, we have seen all the major parties and the Scottish Nationalists being fined for mistakes made on national expenses,” noted Mrs May, adding: “We have paid our fines and I sincerely hope the other parties are paying theirs."

But the First Minister, speaking on the campaign trail in East Dunbartonshire, picked up on the CPS’s observation that while there was no evidence of criminal intent, there might be evidence of inaccuracies in some election returns.

“There is certainly a doubt over the Tories about whether they were really playing by the rules at the last election and there's still some significant questions to answer," said Ms Sturgeon.

In April, the SNP leader suggested Mrs May had called a snap election before the probe "catches up with her" and told the Scottish Trades Union Congress: “We should not allow the Tory Party to escape the accountability for any misdemeanours that may have led to them buying the last General Election."

Her colleague, Pete Wishart, the Nationalist candidate in Perth, said the party backed "urgent reforms" to the elections system.

"A vote for the SNP at this election is a vote to ensure tougher rules and higher standards are introduced," he declared.

Ms Sturgeon added: "One of the lessons perhaps for all of us out of this is do the Electoral Commission need more clout? Because the level of fines they're able to apply are very low and the time might be right to increase that substantially so that all parties know that if they breach these rules then there is a heavy price to pay."