Boris Johnson stands accused of breaking the ministerial code after flying in a jet to campaign in the Hartlepool by-election.
The Prime Minister flew to Teesside International Airport on April 1, five days after the start of the official campaign period for the Red Wall seat.
He travelled by motorcade to a B&Q in Middlesbrough, on government business to promote a rise in the minimum wage, but later met with the then Tory candidate Jill Mortimer for a visit to the local company Hart Biologicals.
The pair then visited a nearby housing estate for door-knocking, leafleting, and to chat with voters.
Government cash should not fund party political work - and any cash spent by a political party supporting a candidate's campaigning has to be declared to the returning officer.
But a document published by Business Insider suggests the Conservative Party declared it spent nothing on transport.
The Tories have claimed the money was accounted for under a different heading, but Labour demanded the Government's ethics adviser Lord Geidt investigate the matter.
Angela Rayner, Deputy Leader, said: “Yet again the Prime Minister behaves like the rules don’t apply to him. Taxpayers’ money should not be abused to fund the Conservative Party’s election campaigns.
“The Prime Minister has clearly broken the Ministerial Code, and this time he can’t play ignorant and pretend that he didn’t know what was going on.
“The contempt with which the Prime Minister treats the laws governing election expenses and the rules that are supposed to uphold standards in our public life shows that he is only ever interested in helping himself, not acting in the interests of the British people."
Electoral Commission guidance underlines that transport costs includes "party members, including staff members (...) around the electoral area, or to and from the electoral area (...) where they are undertaking campaigning on behalf of the candidate."
Parties can spend up to £100,000 in by-election campaigns. According to the return, the Conservatives say they spent a total of £86,991.
The Ministerial Code states that ministers "must not use government resources for Party political purposes." It also says that "where a visit is a mix of political and official engagements, it is important that the department and the Party each meet a proper proportion of the actual cost".
A Conservative Party spokesperson has said that "tours and associated costs" were all "declared in accordance with the rules" and were included on the return under 'Staff Costs'.
"All candidate election expenses were included in the return made in accordance with the Representation of the People Act by the candidate's agent," the spokesman said.
