The National Biosample Centre, in an industrial area between Old Farm Park and Walton Park, has been converted into the country's biggest Covid-19 testing centre .
Scientists there are already receiving swabs from all over the UK and will be increasing capacity every day.
Soon it is hoped tens of thousands of tests will be processed there every week.
The biosample centre, based in a warehouse next to radiator company Pitacs Ltd, was opened by the National Institute for Health Research (NIHR) in 2015, at a cost of £24m.
It is the largest facility in the UK for storing and processing biological samples and has capacity to store more than 20 million samples at temperatures as low as -196C.
Work carried out by its team of experts includes research into to finding treatments and cures for conditions such as dementia and diabetes.
But now much of centre's expertise and technology will be diverted to understanding Covid-19 and testing which people have the virus and how many people have antibodies to it.
This week Secretary of State for communities and local government, Robert Jenrick, confirmed "a warehouse in Milton Keynes" was lined up for this role.
He admitted he didn't know when the site would be fully operational, despite the government facing criticism over its lack of testing.
The test centre plan has been on the books since the third week of March. but it is understood it has been dogged by delays in operating at full capacity.
Universities all over the country received requests to hand over testing equipment to the centre, and staff have worked tirelessly to get the machines up and running.
A spokesman for the Department of Health and Social Care told the Citizen: "The new hub laboratory at Milton Keynes is open, after staff worked around the clock to get this lab up and running from scratch in just a week."
He added: "It is now receiving samples from the new test centres and will be increasing its capacity every day."