Cause for doubt surfaced when it was seen the strain of the disease found in infected cattle was the same as the one used in vaccines at the nearby site.
The building is also shared by pharmaceutical company Meriel and the British Government's Institute for Animal Health. Merial Animal Health is a private US pharmaceutical company.
In response, both have denied any breaches of bio-security.
As of now, the European Commission has imposed an official ban on exports of meat, milk and animals from Britain, though the British government on Saturday had voluntarily halted all exports of cloven-hoofed animals, including live animals, meat and animal products, pre-empting this ban.
Investigations are still on at the Institute for Animal Health Laboratory. Says Merial's managing director David Biland: "'To date our investigations continue to show no breach in our procedures."
"However, it is still too early in this investigation for anyone to determine the cause of the outbreak", he conceded.
In Brussels, a high-risk zone is to be drawn up in cooperation with British authorities. It is expected to include all of the United Kingdom with the exception of Northern Ireland.
The British government had itself requested that the high-risk zone should be extended beyond the area around the farm.
Veterinary experts from the EU states are planning to meet in Brussels on Wednesday to examine whether the ban should remain in place or whether it should be modified or made less stringent.
The Pirbright lab under suspicion handles diseases such as CJD - the human form of mad cow disease. The Institute of Animal Health (IAH) lab is carrying out research on diseases including the highly dangerous variant Creutzfeldt-Jacob disease, avian flu, bluetongue, African horse sickness and salmonella in addition to foot-and-mouth disease.
The outbreak of the foot-and-mouth disease at the Surrey-based farm has led to a call by a Labour MP for a security review of British laboratories, in order to stop dangerous pathogens falling into the hands of potential terrorists.
Alarm about the safety of British labs has been raised by fears that security at the IAH plant or the next-door laboratory run by the US-based private company, Merial, might have been breached.
Professor Martin Shirley, the head of the IAH, in response to a critical report on the labs last year, had admitted "We're trying to deliver a Rolls-Royce service for surveillance in the UK but really we are being funded more and more at the level of a Ford Cortina.''