Daisy Lewis is on her way to Downton Abbey.

According to The Daily Mirror, the British actress (Doctor Who) will play a "prominent" character on Season 4 of this smash hit, coming on board in the role Tom Branson's live-in nanny.

But be warned, fans of Lady Sybill: Lewis may end up taking care of more than just Tom's baby... if you know what we mean!

"Her romance with Tom will be one of the main storylines of the series," a source tells the newspaper. "He will begin to develop feelings for her and they start a secret affair... Tom will have an even bigger role in the new series after becoming the estate manager of Downton."

Downton Abbey Season 4 premieres in the U.S. on January 5.

In other major casting news, Paul Giamatti will appear as the free-spirited brother of Cora.

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.