South Thanet MP Craig Mackinlay has been questioned for several hours under caution by police officers investigating election overspend allegations.
The investigation relates to spending for the May 2015 election, which Mr Mackinlay won with just over 2,000 more votes than his nearest rival, former Ukip leader Nigel Farage.
Mr Mackinlay took 38% of the votes – 18,838 ballots.
The probe by Kent Police was launched following a Channel 4 investigation which questioned the local election spend during the Conservative campaign.
The legal expense limit for South Thanet for that time-frame was £15,016.38. The Conservatives’ local expense declaration was for £14,837.
But a bill of more than £18,000 for stays in two isle hotels and a hostel was not included on local expenses but attributed to national spending. This figure would have taken the party over the legal campaign limit.
Last year Kent Police won a court hearing to have a 12 month extension to investigate the claims.
Mr Mackinlay had contested the move but was defeated.
The penalties for wrongly declaring local elections include the possibility of a year behind bars. If proven it also means the General Election results can be declared void – triggering a by-election.
It is understood police could meet the Crown Prosecution Service on 21 March to discuss bringing a charge in relation to South Thanet.
A Kent Police spokesman 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.”
Mr Mackinlay declined to comment.
Mr Farage has said he may consider standing if a by-election is held for the seat.
Multiple police forces are understood to be investigating 24 MPs in relation to irregularities in election campaign expenses.