Sadiq Khan is facing an official investigation into his conduct after he announced the opening date of the new Crossrail commuter line a day before the local elections.
Transport for London confirmed on May 4 that the new line, which is also known as the Elizabeth Line, would operate in the capital from May 24.
Mr Khan's press office issued a statement on his behalf moments later, expressing his delight “that our world-class new Elizabeth Line will be opening to passengers later this month”.
This prompted Grant Shapps, the Transport Secretary, to complain to the Greater London Authority (GLA).
Typically, “purdah” rules prohibit major political announcements in the weeks before an election.
Writing to Mr Shapps on Wednesday, Emma Strain, the GLA Monitoring Officer, described the allegations as “serious” and confirmed an inquiry would take place into whether the Mayor of London had broken the Members Code of Conduct.
In her letter, Ms Strain said: “Further to my initial assessment, having taken into account the information provided by you and the view of the GLA’s Independent Person, but not having carried out an investigation or made any findings in relation to the complaint, I am of the view that the complaint warrants further investigation.”
The news was confirmed by Mr Shapps in the Commons during transport questions on Thursday morning. It was welcomed by a Department for Transport source.
Mr Shapps told MPs that his complaint related to Mr Khan having confirmed the opening date “in advance of the markets”, and said it would add to the monitoring officer’s “lengthy list of problems”.
While Ms Strain has no legal powers to apply formal sanctions, her decision on whether Mr Khan had broken the code of conduct will be final. There is no means of appeal.
A spokesman for the Greater London Authority (GLA), said: “Having carried out an initial assessment, the GLA’s Monitoring Officer has confirmed that she will now investigate this complaint in accordance with GLA complaints guidance.”
A spokesman for the Mayor of London, said: “The Elizabeth Line is the most significant addition to our transport network in decades, helping build a better city for all Londoners. The Mayor looks forward to the outcome of the Monitoring Officer’s investigation.”
It came as the Evening Standard revealed almost 600 executives at Transport for London earned more than £100,000 last year.
Kay Carberry, the chair of the TfL remuneration committee, said the organisation was “working hard to drive down operating costs” and it was “vital... to be able to attract and retain people with the right technical skills and experience”.