Alan Johnson's ex-bodyguard is facing disciplinary action over allegations that he had an affair with the former shadow chancellor's wife.
The episode led Mr Johnson to quit Labour's front bench last night, forcing leader Ed Miliband to reshuffle his top team less than four months into the job.
Close protection officer Pc Paul Rice guarded Mr Johnson when he was home secretary. He is thought to have worked for Theresa May since she took over at the Home Office last May.
Mr Johnson has refused to elaborate on the "personal reasons" for his shock resignation.
In a statement, he said only that he was finding it "difficult to cope" with "personal issues in my private life".
But responding to reports that Mr Johnson's wife Laura had an affair with Mr Rice, a Scotland Yard spokesman said: "Certain matters regarding a Metropolitan Police Service Pc were referred to the directorate of professional standards.
"We are not in a position to discuss further at this time. No officer is suspended or relieved of duties at this time."
Mr Johnson has been married to Laura, his second wife, for almost 20 years. The couple have a 10-year-old son.
His resignation left Mr Miliband facing his biggest test since being elected Labour leader, forcing him to reshuffle his shadow cabinet less than four months into the job.
He installed Ed Balls as shadow chancellor after overlooking him last October.
Challenged about his previous differences with Mr Miliband over aspects of Labour's economic policy, Mr Balls stressed today that he would be "carrying on" the direction already set.
Speaking on a visit to Warwickshire College in Rugby, he said: "Ed Miliband and Alan Johnson set out a very clear strategy in the last few months which I am carrying on to show that Labour has got an alternative, a responsible, tough but more sensitive way to deal with it which is better for jobs and growth.
"I think that is what people want to hear, that is what I am going to set out."
He was also forced to defend himself from Tory and Liberal Democrat accusations that he was a co-author of Gordon Brown's economic policies.
"I think the reason why a year ago unemployment was coming down was because Gordon Brown and Alistair Darling, in the face of a global financial recession, made the right calls," he said.
"And over the last 13 years did we get everything right? Of course not, but Bank of England independence; not joining in the single currency, investing in the health service, bringing young people into work, those were what people wanted to see in our country.
"And now, to see deep reckless cuts from a Conservative-Liberal coalition which put those jobs at risk, unemployment now rising, I think it's very important that we set out a very clear alternative and that's what I am going to do."
Labour's deputy leader Harriet Harman insisted there would be no difficulties between Mr Balls and Mr Miliband despite their rivalries in the past.
"I think they'll have a very dynamic and productive relationship, with Ed Balls being, I'm sure, an exceptional shadow chancellor under Ed Miliband's leadership," she said.
"He's got a great deal of experience on economic issues."
Mr Miliband has been at pains to stress that there would be no change in economic policy under Mr Balls, who has questioned Labour's deficit reduction plans in the past.
Mr Miliband said last night: "Economic policy is unchanged. Actually Ed and I have similar views."
Conservative Party deputy chairman Michael Fallon said: "Labour maxed out the nation's credit card and now can't agree on how to deal with their debt.
"Ed Balls can't escape his record, and now he can't escape this first question: would he stick with Labour's previous plan and start cutting in 10 weeks time, if so, what would he cut?"
Mr Johnson said nothing to waiting reporters as he arrived for a routine constituency surgery tonight.
He was dropped off outside Hull Central Library and walked briskly through the front entrance without stopping to answer questions.
He had earlier returned to his Hull city centre home after a day of constituency business.