International athletes competing at the London 2012 Olympic Games have started to be tested for banned substances, according to the agency UK Anti-Doping (UKAD), which is in charge of the testing.
Athletes will receive no advance notice of the tests, as UKAD contacts international athletics federations across the world to coordinate its testing programme.
Those competing in the games are banned from taking certain substances which can be performance enhancing, such as anabolic steroids, and samples will be taken for analysis at UKAD's 24-hour laboratory in Harlow, Essex.
UKAD said this will be "intelligence lead" testing, which means "information gathered from a variety of sources, including the analysis of doping risks in each sport and the testing windows for specific substances and methods, will be used to inform activities".
There will be 5,000 anti-doping samples taken for the Olympic Games and 1,250 for the Paralympic Games from a total of 14,700 athletes.
"We are doing all we can to ensure that there is no place to hide for drug cheats in this country," said sport and Olympics minister Hugh Robertson.
"Information sharing across borders and a strong testing programme will help in this fight in the run-up to London 2012 and beyond," he added.
The opening ceremony for London 2012 will take place on 27 July.