Gary Barlow, Mark Owen and Howard Donald are now going forward as a three piece, but they opened up on their new group dynamics on The Graham Norton Show, which will air on BBC One this Friday.
In September, Jason revealed he was quitting Take That and would not be recording on their upcoming album.
Asked about the decision to continue as a band, Mark said: "We had reached a point when we knew we had to do something. We'd been writing all year with the hope Jason would come in at the end. We did everything to try and convince him to change his mind but in the end we had to accept his decision.
"Every day we get a bit more used to it but it still feels a bit weird."
Despite Jason leaving, Howard said the door will always be left open him: "It's a sad thing and the door's always open for Jason to come back in."
The pop group, who will be embarking on a 24-date UK arena tour next year, revealed they had discussed whether they should quit for good but "the support out there kept us going".
Talking about their upcoming tour, which will be in support of their brand new studio album III, Gary said: "We've always put on a big show. This will be no different. Being part of that show is amazing. It's going to be our biggest arena tour ever."
Take That will perform their new single 'These Days' on the show. They performed it for the first time on The X Factor last week, marking their first performance as a three-piece and Gary's return to the talent show following his two-year stint as a judge.
The Graham Norton Show airs on Friday 29 November at 10.35pm on BBC One.
Reveal is now on Google Plus! Join us here for the latest pictures, fashion and celebrity news!
Watch Take That's video for 'These Days' below: