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Boris Johnson should be investigated over a possible breach of the ministerial code after talking about one of his books during an official visit, Labour has said.

The Foreign Secretary was in Serbia on government business last month when he attended a "closed meeting in Serbia's oldest bookstore" which officials claimed was about press freedoms.

Serbian media reports and publisher quotes suggest Mr Johnson "presented his book" on Winston Churchill during the visit.

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".

The opposition party claimed it was a potential conflict between his public duties and private financial interests.

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."

Labour called on Theresa May to clarify whether Mr Johnson had breached the ministerial code, if she had been made aware of his financial interests from book sales from The Churchill Factor before the visit and if she had "agreed action regarding these financial interests".

Shadow minister Clive Lewis said: "Once again, Boris Johnson's conduct has raised questions on his ability to represent Britain internationally, let alone hold the office of Foreign Secretary.

“It is not acceptable that on Armistice Day, Boris used a state visit as an opportunity for self-promotion."