Siobhan Finneran (O'Brien) and Jessica Brown Findlay (Lady Sybil) are also yet to sign contracts for the hit ITV1 drama beyond the third series, according to TV Line.
Maggie Smith, 77, who plays the Dowager Countess of Downton, is "considering her future" on the show, reports The Mirror.
Producer Carnival has denied that there are any problems with contracts, commenting: "We are currently filming series three of Downton Abbey in which all the cast have returned.
"ITV have not yet commissioned a fourth and fifth series which, if they happen, would be a long way off."
However, fellow cast members Michelle Dockery (Lady Mary), Elizabeth McGovern (Cora), Hugh Bonneville (Robert), Joanne Froggatt (Anna) and Brendan Coyle (Bates) have all reportedly signed up for a fourth and fifth series already.
Hollywood's Shirley MacLaine is among the stars who will join the Julian Fellowes show for its third series. Matt Milne, Cara Theobald and Lucille Sharp have also joined the expanded cast list.
Series three of Downton Abbey will air on ITV1 at the end of 2012.
Watch Digital Spy's interview with the cast of Downton Abbey below: