How first a marriage and then a political career were ended

The bodyguard assigned to protect Alan Johnson while he was serving as Home Secretary was suspended last night, following allegations that he had an affair with Mr Johnson's wife.

Scotland Yard confirmed that DC Paul Rice had been withdrawn from service pending a "thorough investigation" into reports of an "inappropriate relationship" with Laura Johnson, the former shadow chancellor's spouse of almost 20 years.

DC Rice, who was part of the Metropolitan Police's Specialist Operations Unit, was made Mr Johnson's close protection officer while he was Home Secretary, a post he held between June 2009 and Labour's general election defeat last May. The detective constable then moved to the team protecting Theresa May when she took the reins at the Home Office.

His professional future now hangs in the balance while he awaits the outcome of the investigation by the police's Directorate of Professional Standards (DPS), to which the case was referred on Thursday, though the absence of any denials is notable.

Unofficially, a source at the Met said: "Everything will hinge on whether or not the relationship started while he was still part of Johnson's protection team and whether he was conducting the affair when he was on duty. If the answer is yes to either of those things then that is career-threatening. But if the relationship started only when he was no longer looking after Johnson then I'm not sure it will be considered a problem."

Last night the force issued a statement which read: "A Metropolitan Police Service Specialist Operations Police Constable has today been suspended from duty pending into allegations reported in the media regarding an inappropriate relationship.

"The matter was referred by the MPS Specialist Operations Unit to the DPS on 20 January. A thorough investigation is now under way. As the allegation is subject to investigation, we are not able to comment further at this stage."

The rumour following Mr Johnson 's sudden resignation was that it was another case of a politician caught philandering, but this was correct only in one limited sense. Like many other politicians before him, Alan Johnson resigned rather than have a sex scandal burst over his head while he was still in office, but, unusually, he was sinned against.

The accusations against the detective constable emerged a few hours after from two tabloid newspapers, the Daily Mail and The Sun, who picked up a tip from the ever-leaky Metropolitan Police that there was an internal investigation of one of Mr Johnson's former close protection officers.

It is thought that both newspapers had been given the tip in exchange for a promise that they would not publish until the officer had been disciplined, so police would not be embarrassed by having to admit that a wayward colleague was still involved in highly sensitive work. Mr Johnson's resignation forced the story out earlier than their informant wanted.

Mr Johnson told Ed Miliband on Monday that he wanted to quit as shadow Chancellor because of problems in his personal life. The Labour leader tried in vain to talk him out of his decision.

To maintain secrecy, Mr Johnson made sure that the circle of people in Westminster who knew what was happening was kept to a minimum. Before he resigned, he rang around the circle of former advisers and others closest to him, so that they would not be taken by surprise when the news broke, but he did not tell even them what had happened in his private life to drive him out of frontline politics.

His departure was met with real regret in Westminster, where he was popular not least because of his background, which is unusual for a politician. Aged 60, he worked for 19 years a postman, and 10 years as an official of the postal union, the UCW, before entering Parliament in 1997. He was untouched by the expenses scandal, and was seen by many – though not himself – as a potential prime minister.

David Cameron said: "Obviously, I am very sorry for Alan because he has given a huge amount of service in public life, on the front line in public life. He's one of the more cheerful and optimistic characters in politics. I am sure he will go on doing a good job for his constituents and being an MP and I hope that he is able to sort out all the things that he wants to. We will miss him on the front bench." Labour's deputy leader, Harriet Harman, described his resignation as "a real loss to frontline British politics".

While it has become relatively common for politicians to be forced out of office because they have been caught cheating on their wives, Mr Johnson is the first to resign because of a looming sex scandal in which he was the aggrieved party.

A year ago, Northern Ireland's political establishment was shocked to find that Iris Robinson, wife of the First Minister Peter Robinson, had had an affair with a 19-year-old man. Mr Robinson stepped down for six weeks while claims that his wife's lover had benefited financially out of the affair were investigated, then resumed office.

A more famous case was that of Harold Macmillan, whose wife Dorothy had a long-running affair with the corrupt Tory peer, Robert Boothby, even having a child by him. Despite the pain this caused her husband, it did not prevent him from rising to be prime minister.

Alan Johnson married Laura Jane Patient in August 1991. They have a 10-year-old son. Friends say the marriage is over. Mr Johnson married before, at 18, in 1968, the same year that he left grammar school to start as a postman. That marriage, from which he had a son and two daughters, ended in divorce.