The former police chief was monitored by a team from Southern Investigations, a firm that has been linked with one of the capital's most notorious unsolved murders.
According to Jonathan Rees, the company's co-founder, the now defunct Sunday tabloid, hired the firm after receiving a tip-off that Lord Stevens was using a Metropolitan Police aircraft to fly up to Northumberland to see a mistress.
There is no suggestion there was any truth in the claim.
It is understood Lord Stevens, who holds a commercial pilot's licence, regularly flew in an aircraft he part owned.
A former Scotland Yard detective, who was working undercover at the firm at the time, has alleged that the private detectives were intent on digging up any embarrassing information they could in a bid to "control" the Met chief, who was in post between 2000 and 2005.
Derek Haslam said he had warned his bosses in the Metropolitan Police that the firm was looking for "filth" on the police and politicians.
He also alleged that the firm was involved in a large amount of criminal activity on behalf of the News of the World.
Mr Rees has denied the claims, but has admitted placing Lord Stevens under surveillance.
He told the website Independentvoices.com: "We were given instructions and an allegation that he [Lord Stevens] was using a Met police plane from Biggin Hill to see his mistress in Northumbria. Now we did organise a surveillance team and we had teams in Northumbria and here, but he never showed up so whether the allegation was true or not, who knows."
The allegations will further tarnish the reputation of the former newspaper that was closed by Rupert Murdoch after the extent of the phone hacking scandal was revealed last summer.
Mr Rees was investigated over the murder of his business partner Daniel Morgan, who was found with an axe embedded in his head in 1987.
It is believed Mr Morgan was investigating allegations of police corruption when he was killed.
No one has ever been convicted of his murder and it has been claimed that the police investigation was influenced by the closeness of the relationship between the News of the World, senior Scotland Yard figures and Southern Investigations.
A spokesman for Lord Stevens denied he had flown a Met police aircraft to Northumbria and denied he had ever had a mistress.
