Every officer of Britain's biggest police force can now be tested for evidence of drug abuse.
Officers of all ranks in the Metropolitan Police can be screened for signs of drugs including cannabis, cocaine, ecstasy and heroin.
In the past, tests were only carried out to vet new recruits and minimise the chances of addicts being admitted to the ranks.
But now, every rank from PC to chief constable, as well as community support officers, specials and civilians who work in special areas, can be tested.
Other police forces are expected to follow suit. Merseyside Police chiefs agreed in December to introduce the new policy.
Officers found to have a drug problem will be offered support, but they could also face disciplinary or criminal action, and ultimately dismissal.
A Metropolitan Police spokesman said: "The use of illegal substances, and the misuse of over-the-counter medicines and alcohol, is proven to impair the judgement and performance of an individual.
"The Metropolitan Police Service has a duty of care to its officers and staff, and the people of London, to deliver a high quality service, safely and efficiently."
The change in policy was introduced on 25 January.