James Norton and Robson Green will return to Grantchester for the show's first Christmas special and a third series next year, ITV said.
James plays sleuthing vicar Sidney Chambers and Robson stars as Inspector Geordie Keating in the quaint Cambridgeshire-set murder mystery series, based on The Grantchester Mysteries by James Runcie.
Morven Christie reprises her role as Chambers' forbidden lover Amanda Kendall in this year's 90-minute festive special.
After leaving her husband Guy (Tom Austen), a pregnant Kendall turns to Chambers for support but worries the clergyman's reputation will continue to throw roadblocks in the way of their relationship.
Viewers are promised a "festive, snow-filled Christmas episode, full of emotion and drama".
Filming has begun on the six-part third series of the show, with the unlikely crime-fighting duo both struggling with temptation.
New Archdeacon Gabriel Atubo will take the lead in reminding Chambers to put his duty above his love for Kendall, while seemingly happily-married Keating will wrestle with love for his wife Cathy (Kacey Ainsworth) and his spark with feisty police secretary Margaret Ward (Seline Hizli).
Former Doctor Who actor Peter Davison will guest star in the series as Geoff Towler, the village solicitor, captain of the cricket team and a man who rules his family with a rod of iron.
Tessa Peake-Jones slips back into the role of sharp-tongued housekeeper Mrs Maguire and faces her own romantic quandary. Her friendship with dashing Jack (Nick Brimble) will start to develop, but she will find it difficult to let anyone get as close as her late husband Ronnie.
Meanwhile timid curate Leonard Finch (Al Weaver) is seeking new romance after being abandoned by Daniel (Oliver Dimsdale).
Diederick Santer, chief executive of producers Kudos, said: "Grantchester continues to be a delight to make and I hope will continue to delight the viewer.
"To be bringing to the audience a third series AND a very special Christmas episode is a particular thrill, and we can't wait to be pouring snow onto the East Anglian Fens."