London, Jan 21 (Agencies): Britain's shadow chancellor Alan Johnson has quit frontline politics after discovering that his wife had an affair with his police protection officer.

Hours after the resignation, Scotland Yard launched disciplinary proceedings against Paul Rice. Reportedly, he had guarded Johnson from potential terror attacks when he was the Home Secretary.

The experienced detective constable, who has been protecting the current Home Secretary Theresa May, is expected to be suspended from duty within 24 hours.

60-year-old Johnson wed his second wife Laura, 47, in August 1991 and they have a ten-year-old son. But last night was reported that the marriage was over.

Shadow Home Secretary Ed Balls has been appointed as the New Chancellor.

Just a year ago, Johnson was being widely tipped as the next prime minister as another botched attempt was being made to topple Brown.

Government sources said the Home Office had been made aware of the affair.

Johnson abruptly announced yesterday that he was facing "personal issues in my private life" and could no longer cope with his high-profile role.

His departure has left Labour in turmoil. Party leader Ed Miliband was forced to give Johnson's
post to Ed Balls, his rival for the top job in September.

Mrs Johnson was at the family's two-storey detached home in Streatham, South London, last night while her husband was in his Hull constituency.

She refused to answer the door but her father John, who lives nearby, said he had no idea of any extramarital relationship. Johnson declined to comment outside a meeting of his local Labour party.

Until last night, Scotland Yard chiefs were also unaware of the role one of its own officers played in the breakdown of the marriage. Johnson thanked Labour leader Ed Miliband for his "support and kindness" Rice is almost certain to lose his job if allegations of serious misconduct are proven.

Police protection officers are carefully vetted for their work, which requires the highest standards of integrity and trust. It is understood Rice worked for Johnson for more than a year as part of his protection team, guarding his home and travelling with him in Britain and abroad.

Johnson would have been a prime target for terrorists and his home, his movements and those of his family would have been closely monitored. In the past, a number of royals, most notably Princess Diana, have been suspected of having inappropriate relationships with some of their protection officers.

But the suggestion that the spouse of a VIP has had an affair with a police bodyguard is thought to be unprecedented.