A Scotland Yard worker has been sacked for abusing a new corporate credit card introduced to stamp out fraud.
Two further police officers are under investigation for suspected misuse of the overhauled expenses system.
Senior officers scrapped the use of American Express cards after an anti-corruption inquiry uncovered fraud and that many officers broke internal rules.
New Barclaycards with lower spending limits, tighter rules and shorter repayment intervals were handed to a smaller number of officers and staff.
But the force admitted an unnamed civilian employee has been forced to resign and was cautioned for theft after buying personal goods through the revamped system.
Officers continue to probe two further cases of suspected misuse and a third case may have been discontinued after the employee left the force.
Caroline Pidgeon, a member of the Metropolitan Police Authority (MPA), said she was shocked by the disclosure.
She said: "After all the publicity about Amex we have got three people already under review with the new card. I find it staggering, quite honestly."
Anne McMeel, Metropolitan Police director of resources, said the inquiries are evidence the new system is picking up potential problems.
She said: "We are looking at this and they are being picked up. If people are misusing them we are picking it up and getting on it.
"No system is absolutely perfect but we are very conscious of the need to make sure this is properly controlled within the organisation."
Documents passed to the MPA also revealed 133 cardholders were already overdue in reconciling a total of £84,199 spent on the cards.
Officials said less money is being spent on the new system because fewer officers hold cards and staff have been told to use other methods to buy equipment.
Commissioner Sir Paul Stephenson ordered a review of spending on American Express cards when colleagues first uncovered evidence of fraud in September 2007.
A total of 3,533 officers and staff were issued with the cards and at one point £3.7 million of public money was unaccounted for.
The majority of this money has been paid back, but legal action is expected against two former officers who owe £82,000 and £1,100.
Hundreds of officers in the force's specialist crime and counter-terrorism wings were stripped of their cards in the crackdown.
Overwhelmed internal investigators discovered so many employees broke in-house rules they agreed a secret amnesty and did not punish them.
More than 300 people were referred to anti-corruption detectives when evidence emerged of potential fraud.
Of these cases, 50 were passed to independent investigators. Three officers have been convicted of criminal offences. Prosecutors are considering 12 more cases.
More than 20 officers have been handed punishments including written warnings, formal reprimands and docked pay.
Inquiries into abuse of the credit card system by officers are expected to continue until next March.
A Metropolitan Police spokesman said the new system was introduced in June last year with "robust processes" to make sure staff use resources "appropriately".
She said: "Among various benefits this has enabled managers and finance units to have greater oversight of expenditure.
"Following an investigation by the Met's Directorate of Professional Standards into the alleged misuse of a corporate credit card a member of police staff received a criminal caution for theft earlier this year. Before receiving this caution he resigned from the Met.
"Local management is currently reviewing the use of corporate credit cards by two serving police officers."