Starring Sirs Ian McKellen and Derek Jacobi with Frances De La Tour and Iwan Rheon, the series focuses on elderly theatrical couple Freddie and Stuart, who live in a flat in London's Soho, and their friends and neighbours.
The new one-off episode will be recorded on Wednesday 23rd December at The London Studios. Tickets for both the recording and a dress rehearsal the previous day are now available
However, whilst ticketing agency SRO Audiences describe the episode as a "series finale", ITV have told British Comedy Guide that the programme is simply a "one-off special".
The final episode of Series 2 saw Freddie and Stuart marry after more than 50 years together.
It had been believed that ITV were unlikely to order a new series after the broadcast of Series 2 in June and July this year, but the new commission suggests they may yet consider further runs of the programme, which has also proved popular in the US.