The panel show is currently on a break in the middle of its 10th series, to allow its regulars and guests the chance to attend the Edinburgh Fringe Festival - as the series is recorded at BBC Television Centre in London.
When the series resumes for the 7th episode of the run on Thursday 8th September, Chris Addison will take a regular position on Hugh Dennis's team, in the seat vacated in January 2010 by Frankie Boyle.
At the same time, Rusell Howard - a regular panellist since the show's 4th series - will not return. He had been absent from the first half of Series 10 in June and July due to existing touring commitments, but had been expected to return in September.
A BBC spokesperson said of Addison's appointment: "Chris is a great signing for the autumn run. With party conferences and major sporting events coming up, I imagine he'll be given a roving role as he has the ability to supply quality ammunition for Dara in the centre as well as the confidence to go it alone. With appearances from the likes of Micky Flanagan, Milton Jones, Ed Byrne, Stewart Francis, Miles Jupp and Ava Vidal in the pipeline, it looks like Mock The Week really will be the panel show to watch this season."
The series returns to BBC Two on Thursday 8th September at 10pm.