Swine flu has now reached Buckingham Palace in London, with two cases being reported from there.
The outbreak, along with a case reported at Windsor Castle, has led to the Queen's order to crackdown on cleanliness at Royal palaces.
The monarch's senior aides have been asked to strictly abide by NHS guidelines, including the isolation of affected individuals to prevent swine flu spreading.
The highly contagious virus has thus far hit employees, rather than members of the Royal Family.
However, according to reports, one of the infected persons at Buckingham Palace is said to work in the catering department.
"Given the speed this thing can spread it was important that anyone with symptoms, particularly around the kitchens, is kept isolated," the News of the World quoted an insider as saying.
"We don't escape catching swine flu just because of where we work and everyone has been taking precautions. The last thing any if us wants is for it to spread to the Queen or the Duke of Edinburgh," the insider added.
The Queen usually spends the week at Buckingham Palace and weekends at Windsor.
However, her aides hope that her risk of contracting swine flu will be reduced by her imminent move to Scottish estate Balmoral, where the monarch will spend the summer.
A Palace spokesman said: "As a large and diverse organization, employing over 1,000 personnel in a number of locations, we accept there are likely to be cases of suspected swine flu. Managers have been briefed to take sensible precautions."
Earlier this week, Prince Andrew had to scrap a visit to a factory because of a suspected swine flu outbreak.
He was due to officially open a new factory for digger maker JCB in Uttoxeter, Staffs, and present the 40million-pound plant, JCB Heavy Products, with a Queen's Award for Enterprise.