Mr Johnson had faced some criticism for his handling of the shadow chancellor’s brief A Metropolitan Police specialist operations officer has been suspended from duty following allegations he had an inappropriate relationship with the wife of Labour politician Alan Johnson.
Mr Johnson stood down unexpectedly as shadow chancellor on Thursday, citing “personal reasons”.
Newspaper reports have suggested that his close protection officer, Pc Paul Rice, had an affair with his wife.
The Met Police’s standards directorate is investigating the allegations.
In a statement on Friday evening, Scotland Yard said: “A Metropolitan Police Service Specialist Operations Police Constable has today, Friday 21 January, been suspended from duty pending the outcome of the Directorate of Professional Standards (DPS) investigation into allegations reported in the media regarding an inappropriate relationship.
“A thorough investigation is now under way. As the allegation is subject to investigation, we are not able to comment further at this stage.”
It is understood that Pc Rice was assigned to protect Mr Johnson, 60, and his wife when he was home secretary in the previous Labour government.
Scotland Yard has not confirmed the name of the suspended officer.
Mr Johnson’s decision to quit his shadow cabinet job came as a complete surprise at Westminster, even though he had suffered some criticism for his grasp of its more technical aspects.
He said he had “found it difficult” to cope with the responsibility at the same time as dealing with issues in his private life.
He will be replaced by Ed Balls, who in turn hands over the shadow home secretary’s brief to his wife, Yvette Cooper.
Mr Johnson, who will remain as MP for Hull West, has refused to comment further on his resignation and has not given any interviews.
The former postman and trade union leader has been married to his second wife Laura for almost 20 years and the couple have a 10-year-old son.
Labour leader Ed Miliband told the BBC he had accepted the resignation “with great regret” and Mr Johnson’s reasons for standing down had “nothing to do with the job” of shadow chancellor.
In an interview earlier this month, he appeared not to know the rate of National Insurance paid by employers, and he was also reported to have clashed with Mr Miliband over the policy of introducing a graduate tax to replace university tuition fees.
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