A senior detective attempted to steal £160,000 confiscated by police like a "seasoned fraudster", a court heard.
Detective Inspector Christopher Spackman told two men suspected of obtaining the money illegally that they could not have it back.
And he tricked a Hertfordshire Police accountant into giving him a cheque made out to one of the men - and then tried to cash it, the Old Bailey was told.
Married Spackman eventually got the money paid into a girlfriend's account and they spent some on designer clothes and a deposit for a car, said Crispen Aylett, prosecuting.
But the scam had been spotted by other officers and Spackman was arrested with his girlfriend, Joanne Fletcher, and accomplice Trevor Powell.
Spackman, 47, who worked at Borehamwood Police Station, and Powell, 46, of
Watford, pleaded guilty to conspiracy to steal.
Spackman, of Rickmansworth, Hertfordshire, and Fletcher, 32, of Aldernham, near Watford, pleaded guilty to theft. Spackman also admitted misconduct in office.
Mr Aylett said Spackman had been determined to get his hands on the money which was unclaimed in a police bank account.
Mr Aylett said: "The lengths he went to, the lies he told and the documents
that were forged, would have been worthy of a seasoned fraudster."
The court was told £160,114 in foreign currency was seen passed between two cars at the South Mimms service station on the M25 in Hertfordshire, in March 2001.
New equipment
It was thought to have been obtained dishonestly and was confiscated with a mobile telephone and papers from driver John Maher, of Rugby.
The Crown Prosecution Service later decided there was insufficient evidence to prosecute.
The court heard that when Mr Maher and the other driver asked for their property to be returned, they were told by Spackman that they could not have the money.
"He said the money would be used for new equipment such as police cars," said Mr Aylett.
Spackman and Powell had made inquiries about opening a building society account for the money and obtained documents in Mr Maher's name.
When they realised they could not get away with it, Spackman returned the
cheque in Mr Maher's name and concocted another story.
A second cheque was made out to a fictitious name and paid into an account controlled by Fletcher, said Mr Aylett.
They were arrested after spending just over £3,000. Police recovered £156,985.
Undercover operation
Mr Maher and the police accountant had been in on the police undercover operation and some of their conversations had been taped.
After their arrest in October, last year, Fletcher said Spackman had been
obsessed with her but their relationship had not been sexual.
Spackman said he had taken the money mainly to help Fletcher pay for medical treatment and set up in business as a beautician.
Powell said he had dropped out of the plot when he realised it would not work.
The sentencing was adjourned until Thursday.
Spackman was remanded in custody while Fletcher and Powell were granted conditional bail.
But Judge Brian Barker, QC, told them: "All options are open."