Another blow for Theresa May just before the General Election. The Conservative Party used dirty tricks to prevent Nigel Farage being elected to Parliament in 2015. They are now facing criminal charges for election expenses fraud.

The Conservative candidate for South Thanet, a member of his staff and a Tory election organiser have been charged over allegations of illegal election spending during the party’s 2015 campaign to win the seat.

The Crown Prosecution Service said there was sufficient evidence to charge three people, among them Craig Mackinlay, who was the MP from 2015 and is seeking re-election on 8 June. Nathan Gray, his election agent, and Marion Little, a party organiser, were also charged, the CPS said.

Mackinlay will remain on the ballot for next week’s election as nominations have closed and some postal votes will already have been submitted.

Speaking on a campaign trip to Doncaster, Theresa May said: “The Conservative party continues to believe that these allegations are unfounded. Craig Mackinlay is innocent until proven guilty and he remains our candidate.”

Mackinlay, who was also backed by his local Conservative association, said it was “a shocking decision by the CPS” and he expected to be acquitted.

“I will not let this decision affect the hard work I do for my constituents and the hard work I hope to do for them after 8 June,” he said.

“Our justice system is underpinned by the presumption of innocence and I am confident that I will be acquitted, as I have done nothing wrong and acted honestly and properly while a candidate in 2015, and as all candidates do, acted upon advice throughout.”

A separate Conservative statement said the party was standing by Mackinlay, and referred to previous allegations against candidates, over which the CPS opted to take no action, as “politically motivated”.

This prompted the Labour leader, Jeremy Corbyn, to say the Conservatives should be “extremely careful” about making such comments.

In 2015, Mackinlay secured a majority of just over 2,800 in South Thanet, beating the then Ukip leader, Nigel Farage, following a tense campaign. Farage is not contesting the seat again, with a local councillor, Stuart Piper, representing Ukip.

Nick Vamos, the CPS’s head of special crime, said: “On 18 April we received a file of evidence from Kent police concerning allegations relating to Conservative party expenditure during the 2015 general election campaign.

“We then asked for additional inquiries to be made in advance of the 11 June statutory time limit, by when any charges needed to be authorised. Those inquiries have now been completed and we have considered the evidence in accordance with the code for crown prosecutors.

“We have concluded there is sufficient evidence and it is in the public interest to authorise charges against three people. Craig Mackinlay, 50, Nathan Gray, 28, and Marion Little, 62, have each been charged with offences under the Representation of the People Act 1983 and are due to appear at Westminster magistrates court on 4 July 2017.”

Since criminal proceedings were active, Vamos added, it was important for people to not speculate or comment about the case, which could potentially prejudice it.

The charges follow allegations of overspending by some Conservative candidates and their agents at the 2015 election.

Last month, the CPS announced that no criminal charges would be brought against more than 20 Conservative MPs over the national party’s failure to accurately declare expenditure on a battlebus tour.

The CPS said their constituency spending declarations “may have been inaccurate”, but concluded that there was insufficient evidence to prove dishonesty or bring a criminal case against the MPs and their agents.

At the time, the CPS said the one case outstanding related to Mackinlay and it would announce the decision soon. He was interviewed under caution by police in March over the allegations.

Mackinlay faces two counts of having knowingly contravened the 1983 Representation of People Act over election expenses. Gray faces three charges over the same law in relation to his role as Mackinlay’s agent. Little faces five charges, two connected to aiding and abetting Mackinlay in his alleged wrongdoing, and three connected to Gray.

The three could be tried at a crown court. The maximum sentence is one year in prison or a fine.

A Conservative party spokesman said: “The legal authorities have previously cleared Conservative candidates who faced numerous politically motivated and unfounded complaints over the party’s national battlebus campaigning.

“We continue to believe that this remaining allegation is unfounded. Our candidate has made clear that there was no intention by him or his campaigners to engage in any inappropriate activity. We believe that they have done nothing wrong, and we are confident that this will be proven as the matter progresses.”

Warning that the media and others should beware of prejudicing the case, the Conservative statement added: “There is a broad consensus that election law is fragmented, confused and unclear, with two different sets of legislation and poor guidance from the Electoral Commission.

“Conservatives are committed to strengthening electoral law to tackle the real and proven cases of corruption that were exposed in Tower Hamlets in 2015.”

In response, Corbyn said: “Nobody should be commenting on the details of an ongoing case. The police must be allowed to act independently to investigate on the basis of any evidence they’ve got, and the Crown Prosecution Service must be allowed to make its decision on whether to proceed on a case.

“It is a very bad road when democratically elected politicians start offering a running commentary on independent judicial processes. We have to have total separation of political and judicial powers in this country.

“All politicians need to be extremely careful – politicians are elected to parliament to be held to account by the public.”

Voters in Ramsgate believed the CPS decision would have little effect on the outcome of the vote. “You could put up an orangutan in this constituency and the Tories would still get in,” said Les Lewis, 63, a retired market trader and Labour voter. “A few years ago this was the second or third poorest ward in the country and people still vote Tory, so don’t expect this will make a blind bit of difference.”

Others said they could see little alternative, choosing to see their ballot as support for the party as a whole rather than the local candidate. Asked about Mackinlay’s situation, Joyce Bright, 80, a retired NHS purchaser, said: “To be honest, what they say is quite good, but money doesn’t grow on trees and they think they can just magic money out of nowhere to pay for their policies.”

Meg Chater, a retired textile artist, said she felt deprived of a democratic right. “I’m appalled,” she said in relation to the timing of the charges. “I think we have been robbed of a candidate. He shouldn’t have stood and they shouldn’t have let him stand knowing this was hanging over him. I vote Conservative and now I feel I’ve been robbed of a candidate and have nobody to vote for.”

Leslie Sandles, a market trader, believed the CPS were right to charge even with six days to go before an election.

Asked about the matter during a campaign trip to the Clacton constituency, Farage said it was “bad judgment” from May to support Mackinlay. “Why on earth would you allow someone to go ahead as general election candidate when this cloud was clearly hanging over him? There will be questions,” he said.