Labour's candidate for West Midlands Mayor has been accused of diverting public money intended for parliamentary duties into his campaign, a move which would breach Independent Parliamentary Standards Authority (IPSA) regulations.
Tory MP Andrew Bridgen has written to IPSA demanding an inquiry into Mr Byrne's "alleged gross misuse of parliamentary expenses".
In the letter he calls for an inquiry, saying that it was unclear where the "huge resources" that Mr Byrne had put into three political campaigns since 2019 – two of which were non-parliamentary – had come from, "if not the taxpayer".
The allegations come just two weeks before the election, where Mr Byrne will be the main rival to current Conservative Mayor Andy Street.
Labour said expenses were recorded in line with regulations and that all campaign donations were "within the rules".
The letter includes reference to a claim from an unnamed local election candidate who reportedly said Mr Byrne's parliamentary staff were being made to "work full-time on the mayoral election".
It also includes an allegation said to be from a former staff member of Mr Byrne's, who complained to Labour's ruling National Executive Committee that he was "using his publicly funded parliamentary staff to run his mayoral campaign".
According to the letter, the staff member claimed their own wages were paid from Mr Byrne's parliamentary budget, despite doing work that was "100 per cent non-parliamentary".
A further allegation says Mr Byrne employed a "constituency assistant" through the House of Commons, who described his role primarily as "managing" his "campaigns". He is also said to have employed a parliamentary head of research who "developed local policy" for his Mayoral campaign.
Leicester MP Mr Bridgen's letter also says Mr Byrne has spent more than £31,000 on Facebook advertising since the end of 2018, but has only registered donations of £23,583.20 during that time.
Mr Bridgen wrote: "I would contend that these are serious breaches of the regulations published by IPSA and I ask that these be investigated as a matter of urgency."
A spokesperson for the Labour Party said: "All expenses are recorded in line with IPSA's regulations and all campaign donations are within the rules and will be reported in the usual way."
Birmingham MP Mr Byrne is bidding to become West Midlands Mayor at next month's election. He is up against Conservative incumbent Andy Street, Steve Caudwell for the Greens, Lib Dem Jenny Wilkinson and Pete Durnell from Reform UK.