The prime minister’s low-key visit to the Devon constituency that has voted for the Tories since its creation comes amid concern the seat could fall to the Liberal Democrats when voters head to the polls on 23 June.
The by-election was triggered after the resignation of the former Conservative MP Neil Parish, who became the focus of a political storm after admitting watching pornography on his phone in the Commons chamber.
Despite the largely rural seat voting overwhelmingly for the Tories at the 2019 election — the party’s majority was over 24,000 — officials are concerned the seat could fall to Sir Ed Davey’s party in a fortnight.
The Lib Dems are seeking to capitalise on the ongoing concern over the prime minister’s precarious position and deep anger over the Partygate scandal that resulted in Mr Johnson being fined by the Metropolitan Police.
A government source confirmed Mr Johnson’s visit to Tiverton and Honiton on Thursday, but the prime minister is not expected to participate in any media interviews.
Earlier Mr Johnson was pictured making a surprise visit to the Royal Cornwall Show alongside the environment secretary George Eustice, just hours before Prince of Wales and the Duchess of Cornwall were due to arrive.
With Mr Johnson’s position in No 10 under the spotlight after Monday’s confidence vote, a defeat in Tiverton and Honiton in 13 days’ time could reignite calls for his resignation.
Earlier this week the Conservative candidate in the Tiverton and Honiton by-election refused to say how she would have voted in the no confidence ballot on Mr Johnson’s leadership had she been an MP.
Helen Hurford, a former headteacher, described the question as “irrelevant”, saying she is not “in Westminster”, adding it was time to “move forward” after the prime minister narrowly survived the vote on Monday.
On the same day, a second by-election will be held in Wakefield — triggered after the resignation of Tory MP found guilty of sexually assaulting a 15-year-old.
The former Labour stronghold voted for the Conservatives at the 2019 general election, but is widely expected to be regained by Sir Keir Starmer’s party on 23 June.