A Conservative MP has been interviewed under caution as part of an ongoing police inquiry into whether the party overspent in its campaign for South Thanet in the 2015 general election, when they were up against Nigel Farage.

Craig Mackinlay, the MP who won the seat against the former Ukip leader, is said to have spent about six hours speaking to police about their investigation, which has been ongoing for about a year.

Asked about the interview, a Conservative spokesman said: “We are cooperating with the ongoing investigations.” Mackinlay did not reply to a request for comment.

There is growing panic in the Conservative party about the scale of police probes into election spending, which could affect dozens of MPs. A separate investigation by the Electoral Commission into whether the national party broke election spending limits is also under way and expected to come to a head within weeks.

The allegations, first uncovered by Channel 4 News and the Daily Mirror, are that spending in marginal seats on a battlebus tour and teams of party officials was wrongly recorded as national, rather than local spending.

The penalties for wrongly declaring local elections are steep, with possible criminal charges for MPs and their election agents, and results can be declared void.

It is understood police could meet the Crown Prosecution Service as early as 21 March to discuss bringing a possible charge in relation to South Thanet, where Farage was narrowly beaten by Mackinley.

Farage, the former Ukip leader, has already said he may be interested in rerunning in the Kent coastal seat if it there were to be a prosecution and byelection.

Kent police said: “The investigation into this complex matter is ongoing and officers continue to follow lines of enquiry. Therefore it would not be appropriate to comment further.

“Officers from Kent police continue to work with the Electoral Commission as the investigation continues.”

Separately, a group of Conservative MPs under investigation over their election expenses are growing increasingly frustrated at the lack of support from the party’s headquarters.

One has sent an email to Tory HQ accusing the party of keeping secret a draft of the Electoral Commission report from MPs whose local spending returns are under investigation.

In an email seen by Sky News, Karl McCartney, a Tory MP under investigation who is helping other MPs, accused party officials of trying to save themselves rather than help those who were elected.

He wrote that his colleagues “feel completely cast adrift by CCHQ/whips/the parliamentary party and left to fend for themselves”.

He added: “At what stage do you think you [the party] might inform us that another media s***storm is coming? We didn’t create this mess, the clever dicks at CCHQ did, and I don’t see their professional reputations being trashed in the media much.”

“The initial cock-ups, ‘strategy’ and ineptitude with regard to this issue that has so negatively impacted our: lives, standing in our communities, standing amongst colleagues, families and our regard for particular parts of the party centrally, and were all of CCHQ’s making … need to stop.

“We are the ones who are now [and since the beginning as individuals have been] in the media spotlight and it might have been a little more reassuring and collegiate if the powers that be in our party perhaps tried to be a little bit more supportive and less interested in covering their own backsides.”

A Conservative source said the party did not hold a copy of the Electoral Commission report as McCartney alleged.

Speculation has been rife in Westminster that a series of byelections could be triggered by police prosecutions over spending or potentially encourage Theresa May to call a general election to avoid an embarrassing focus on the constituencies involved.

It is understood the possibility of a byelection in South Thanet was discussed recently at a meeting involving the Labour leadership team, with a view to making sure the party was prepared. The seat was held by a Labour MP, Stephen Ladyman, from 1997 until 2010, but the party came third at the last election behind the Tories and Ukip.

Mackinlay held Farage off by 2,812 votes and declared spending of £15,016, just below the limit of £16,000. Under election laws, any costs incurred to promote a candidate must be declared on local candidate spending returns.

Receipts for national Conservative party spending, however, show that

£18,000 was spent on accommodation for party workers in the Royal Harbour hotel in Ramsgate, in the constituency, and a Premier Inn in Margate, which is just outside it. A further £715 was spent on the Alpha hostel, used to accommodate a busload of activists who took part in a Stop Farage campaign.

Among the Conservative party officials based in South Thanet during the campaign was Nick Timothy, who is now one of Theresa May’s joint chiefs of staff.

Questioned previously by police about the spending in a letter, Mackinlay told the investigating officer: “I can confirm that nobody staying at the hotel was subject to my, or my agent’s, control or direction, any national Conservative party staff based in the Royal Harbour Hotel were part of a national campaign team and were engaged in activities at the direction of Conservative central headquarters.”

He has previously said the party maintains that the South Thanet general election return was “both lawful and proper”.