Smith, 77, who plays Dowager Countess of Grantham in the period drama, hasn’t yet put pen to paper to secure her place on the show.

Bosses of the show, which is filming its third series, are looking ahead to the fourth and maybe fifth series.

Michelle Dockery, who plays Lady Mary, and Hugh Bonneville, who plays Earl Grantham, are both reported to have signed contracts for upcoming seasons.

Elizabeth McGovern, who stars as Cora, Countess of Grantham, Joanne Froggatt, who plays housemaid Anna and Brendan Coyle, who plays valet John Bates, have all said they are returning.

But Smith and Dan Stevens, who plays Matthew Crawley, have both yet to sign, leaving bosses concerned that they’ll quit the show.

A source said: “Downton Abbey has a history of major surprises, and the producers are not opposed to changing casting within the show to add to the drama.

“But it’s hard to imagine the series without Maggie Smith or Dan Stevens involved in some way.

“To write the Dowager Countess out of the series, or even kill off the character, would be Downton’s boldest storyline yet.”

Heartthrob Dan Stevens is expected to marry Lady Mary in the next series after their engagement was announced during the Christmas Day special, but bosses are keeping schtum about their plans.

Bosses insist that there’s no cause for concern over the show, which will see its third series hit screens this autumn.

A spokesman for producers Carnival said: “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.”