A probe should be launched into whether Boris Johnson breached ministerial code by promoting his book while on an official visit to Serbia, Labour has demanded.

The Foreign Secretary was in the country on Government business last month when he attended a “closed meeting in Serbia’s oldest bookstore” in Belgrade that officials emphasised was about press freedoms.

But according to Serbian media Mr Johnson “presented his book” his book the Churchill Factor – a biography of the wartime Prime Minister Winston Churchill written before he was appointed to his role by Theresa May.

The Government’s code of conduct declares ministers must ensure “that no conflict arises, or appears to rise, between their public duties and their private interests”.

Labour said the publicity around the event appeared to "contradict comments from the minister's office that he had been invited to discuss freedom of the press and was not aware the event would be used to promote his book" and claimed it was a potential conflict between his public duties and private financial interests.

But an FCO spokeswoman said: “As we made clear at the time this was a meeting with a small group of Serbian editors on media freedoms. It was in no way a promotional event.

“We don’t recognise the claims made by Tanjug newspaper who did not attend the meeting".

In a letter to the Prime Minister, Andrew Gwynne suggested the event raised questions over Mr Johnson's future in the job.

The shadow minister without portfolio wrote: "As you are aware the ministerial code sets out strict rules forbidding any such conflicts and further states that it is the personal responsibility of ministers to avoid conflicts of interests.

"Should it not be possible to avoid conflicts of interest, the ministerial code states that the Prime Minister must be consulted, and that it may be necessary for the Minister to cease to hold the office in question.

"In light of the Foreign Secretary's conduct during a ministerial visit to Serbia, I request that the incident be investigated to determine as soon as possible if a breach of the ministerial code has taken place."

A spokesman for Johnson told the Guardian: “The Serbian newspaper making the suggestion this was a promotional event is simply wrong and wasn’t even at the event. We have said before that this was not a promotional event and we made that request very clear in advance of the trip.

“The event took place in a closed bookshop and focused on press freedoms. Maybe Labour should focus on coming up with some policies for the good of the country instead of wasting everyone’s time on this kind of rubbish.”