A spokesman for QinetiQ said, "We have successfully simulated searches at the level of about a million people per hour in our effort to stress the site to ensure that the load will hold up.
"We have also done a series of automated concurrent searches that have seen the equivalent population of a small town being able to undertake a typical variety of searches of the database at the same time."
However, QinetiQ was unable to provide a date for when the site is likely to be relaunched. The spokesman explained, "There is no actual date for the site going live at the moment but we are working with the PRO to move it forward."
The site, which accesses the details of 32 million people who lived in England and Wales in 1901, was taken down earlier this year after struggling to cope with unprecedented numbers of visitors. It was designed to cope with up to 1.2 million hits a day, but on the day of its launch in January the site was receiving more than one million hits an hour.
QinetiQ, which was formed from the Defence Evaluation and Research Agency, has highlighted some of the complexities involved in a project of this scale.
The spokesman stressed, for example, that the 1901 census site is not simply a Web site, it is a complex database system that is accessed via the Internet. He said, "Some people thought that because it was perceived as a Web site it would be possible to mirror or cache it onto other servers, but that is not possible because it is a complex database.
"All the work that we are doing is to make the site far more robust and ensure that visitors receive a fulfilling visit."