The 24-year-old actress hired the maid to work in her New York apartment at the beginning of last year but Border Agency staff have since launched a probe following a complaint that the woman was travelling to London on and off to help Watson in her Islington home, reported Daily Mirror.
It is believed that the housekeeper spent a total of three months in Britain between September last year until early February and was quizzed on arrival by immigration staff about her ability to pay for her stay.
Although the home office can't comment on individual cases a source from the department said, "An allegation has been received and processed and has been allocated for further action."
Meanwhile, Watson recently graduated from the prestigious Brown University in the United States.
The 'Bling Ring' actress, who enrolled at the Ivy League school in 2009, received her English literature degree at a commencement ceremony on the main campus in Providence, Rhode Island, last month.