Police have interviewed a Tory MP under caution for six hours over election expenses amid a growing “s***storm” over wider allegations relating to the party’s 2015 general election spending.

Craig Mackinlay, the Conservative MP for South Thanet, who defeated the then Ukip leader Nigel Farage and comedian Al Murray at the election in 2015, reportedly spent several hours speaking to police about their investigation into his seat.

According to the Telegraph, police would not comment on Mr Mackinlay’s interview but added: “The Kent Police investigation into this complex matter is ongoing and officers continue to follow lines of inquiry”.

“Therefore it would not be appropriate to comment further. Officers from Kent Police continue to work with the Electoral Commission as the investigation continues”.

It comes after the Electoral Commission opened a probe following an investigation by Channel 4 News that found the Conservatives transported busloads of volunteers to marginal seats to aide with the campaigning, but that the expenses, including travel and accommodation costs, were not registered on local spending reports.

The so-called Battlebus 2015 tour saw Tory activists driven to 29 marginal seats in the days before the general election. It has been reported that Downing Street officials have been “deeply worried” about the ongoing police investigation into the seats of more than a dozen MPs. If the election result in a seat is declared void, a by-election could be triggered.

A Conservative spokesman repeated the line that party HQ is “cooperating with the ongoing investigations”.

It also came as leaked emails, seen by Sky News, claimed MP Karl McCartney, whose seat is one of over a dozen being investigated by police, wrote to the party chairman Patrick McLoughlin expressing dissatisfaction over the handling of the “s***storm”.

He added: “We didn’t create this mess, the clever dicks at CCHQ (Conservative Campaign Headquarters) did and I don’t see their professional reputations being trashed in the media”.

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.