Blue would break-up if one of their members decided to leave for good.
The 'All Rise' hitmakers - who reunited in 2011, six years after going their separate ways - still speak to one another every day and Antony Costa has insisted that he, Simon Webbe, Duncan James and Lee Ryan wouldn't feel comfortable performing as a three-piece group.
Antony said: "We still speak to each other every day. We've got our WhatsApp group and have a bit of banter and a laugh.
"Blue is a four-piece - it will never be a three piece. If one member left that would be the end of Blue. We're very much brothers for life. It sounds a bit corny but we've been in this business for 22 years now."
The group haven't released new music since their 2015 album 'Colours' and although they don't have any gigs in the pipeline, Antony insisted that doesn't mean they are over.
Asked about future performances, Antony said: "Never say never. We've got to a stage now where we go off and do our own thing and then come back together and do a few gigs over the summer but there are no talks at the moment.
"Music has changed now, so if there's a call for it then great, but if not, we'll just plod on and do our gigs."
The 'One Love' singers met while auditioning for other groups and Antony credits himself for pulling together their complete line-up.
He told new! magazine: "Basically we were all mates.
"I knew Lee and Dunc on the audition scene. You'd go for different boy band auditions and always see the same faces. Me and Dunc got really close and you exchange numbers and wish each other luck and then Lee came along and that was it really.
"We got together and Simon came later because he knew Lee and Dunc. But he'd never met me so it was down to me to sort of say, 'Yeah I think Simon's a good lad and he'd be great in the band.' And that was it - that's the story in a nutshell."