Bosses behind the major British period drama Downton Abbey - which has also proved a huge hit stateside - have moved quickly to tie down the show's biggest stars in a bid to avoid another Dan Stevens-esque exit.

According to The Sun, stars including Hugh Bonneville and Elizabeth McGovern have signed three year deals worth over £1 million each. The move was made to avoid another cast member leaving in the same way that Stevens did last year.

The 30-year-old, who played Matthew Crawley, was the ITV drama's major draw though left abruptly to pursue other interests. The actor is a keen fan of literature and has narrated several audiobooks including The Angel's Game, Wolf Hall and War Horse. In 2012, he was a member of the judging panel for the Man Booker Prize for Fiction and is currently editor-at-large for The Junket, an online quarterly is co-founded.

Stevens' character was killed off in the show's 2012 Christmas special - leaving audiences bemused - and bosses are not keen on a repeat.

"Downton's charm is tradition so bosses were desperate to secure the show's big names. All the major characters are now on big deals," a source the British tabloid, "It was also important to keep Allen [Leach, who plays Tom Branson]. His role has grown considerably since he started as the chauffeur and you will see him more and more."

Leach's character is thought to be instrumental in the upcoming fourth series of the show, in which Lady Mary is trying to move on from the death of her husband Matthew. 

"The relationship with Tom and Mary gets stronger as they're unified with their grief," the source continued, "Tom understands what she's going through and tries to help her through that mire of grief and sorrow.The show for Tom this year sees him find his place within that household."

In its new season, Downton Abbey moves into the roaring twenties and with it comes the arrival of several new characters, including a charismatic and charming American jazz singer played by Gary Carr. Dame Kiri Te Kanawa, Julian Ovenden, Joanna David, Dame Harriet Walter and former Eastenders actor Nigel Harman have also joined the show.

Downton Abbey returns on ITV next month.

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