Scotland Yard says John Prescott will not face a criminal investigation over allegations he had sex in his office with his secretary Tracey Temple.
Ms Temple (pictured with Mr Prescott in 2001) has sold her story
The deputy prime minister admitted the affair but said many of the details published in a newspaper were untrue.
A retired police officer had made the complaint against Mr Prescott.
Alistair Watson wrote to the Met's Commissioner Sir Ian Blair citing the case of a police officer punished after he had sex with a woman while on duty.
Ms Temple, Mr Prescott's former diary secretary, claimed they had sex at his Whitehall office while they were supposed to be working.
Responding to Mr Watson, Deputy Assistant Commissioner John Yates wrote: "Misconduct in public office is a serious offence. The threshold around misconduct is a high one and there must be a serious departure from the accepted standards.
"In addition, the consequences likely to follow are relevant when determining whether the conduct amounted to an abuse of the public's trust or not.
"It is considered that the potential consequences in respect of the alleged behaviour, even if proved, would not be so serious as to call for a prosecution."
He added that a distinction had to be drawn between action that could potentially discredit "an office holder and those actions that constitute criminality".
"Accordingly I have decided not to embark upon a criminal investigation. To do so would not I believe be a proportionate response or an appropriate use of police resources."
Mr Watson attacked the ruling, adding: "This is truly shocking and disgraceful and reprehensible that that man is not going to face a criminal investigation.
"The police officer from Manchester did exactly the same thing and he's not been able to say: 'I'm sorry, it was all a private matter'."
London Assembly Tory policing spokesman Richard Barnes accused the Met of "appalling double standards".
He said: "I'm not sure Prescott should be hauled into the court, but it is not acceptable that he has escaped any real punishment when two rank and file police officers lost their jobs, pensions and got criminal records for doing a similar thing."
Extracts of Mr Watson's letter, published in the Sunday Times, said: "I think in the interest of equality and justice, there is no reason why Mr Prescott and Miss Temple cannot be prosecuted.
"This is not a malicious thing. If there are rules that apply to ordinary people, somebody like John Prescott should be treated the same, or more harshly."
He also said Ms Temple should forfeit any profits made by selling her story to a newspaper to help "deter others in public employ".
In last week's post-elections Cabinet reshuffle John Prescott was stripped of his department, although he was kept on as deputy prime minister and allowed to retain his salary and grace-and-favour homes.
The decision to allow Mr Prescott to retain his ministerial perks provoked anger among some opposition and Labour MPs.