The 78-year-old, who starred as Manuel in the 12 episodes of Fawlty Towers, will appear in the two-parter as a priest called Father Morgan, alongside ex-EastEnders actor Joe Absolom, who plays loan shark Benny.
With the intention of moving to the States to start afresh, Benny arranges to collect his final lot of debts from Father Morgan and a poverty-stricken family.
Benny suddenly has a change of heart, however, when he turns to the priest for help, which also leads to help with a crazy scheme which leaves them both in a compromising situation.
Series producer Oliver Kent said: "Andrew Sachs is a living legend. When Andrew was suggested earlier in 2008, we leapt at the chance to have him in our special double Christmas episode. Andrew’s performance as the ageing and kindly priest, Father Morgan, is very touching."
The episodes - entitled 'This Will Be Our Year' and 'Took A Long Time To Come' - were filmed in mid-September and are scheduled to be broadcast on Saturday 20 and Sunday 21 of December.
Last month, Sachs became embroiled in the Jonathan Ross and Russell Brand phone scandal, dubbed 'Manuelgate'. Click here to read DS's in-depth coverage.
Over the weekend, Sachs was reunited on stage with Fawlty Towers co-star John Cleese for the first time in nearly 30 years for Prince Charles's 60th birthday celebrations.