The band are preparing to embark on a 36-date run of stadium shows from Friday (27.05.11) and the singer admits he has been stricken with nerves, but is now looking forward to the concerts.
He said: "A few weeks ago I had a pre-tour panic, a bit of a meltdown, which I went through for a couple of days.
"Not much sleep, but I came through the other end and I've been all right since.
"But we're confident with it now and I think the show's going to be brilliant."
The 'Progress Live' tour is the first series of concerts the band have embarked on since Robbie Williams rejoined the band last year, 15 years after he quit, and Mark admits they were initially worried about how he would cope not performing solo, but the 'Rudebox' singer has turned out to be the most enthusiastic about their elaborate stage productions.
Mark added to Music Week magazine: "Rob's had to come in and do things he hasn't done for 10 years or so, and be part of a group. It's a whole new thing for him, but seeing how well he's adjusted to it is great. There was a feeling like, 'What if he doesn't want to put clown make-up on while he's singing' or 'How's it going to work with all of us?' but he's definitely part of the band in the truest sense.
"In fact, we have to hold him back now - he wants to put make-up on during every song!"
Though Robbie has been back in the band - which also comprises Gary Barlow, Jason Orange and Howard Donald - for some time, Mark admits he is still taken by surprise when he realises they are a five-piece once again.
He said: "Now and again I'd catch myself looking over my shoulder thinking, 'Oh, we're back, all of us' and I'd laugh inside. I can't believe it. I keep getting this warm feeling."