And with their popularity at an all time high, the question many fans were asking was, "why?"
The remaining members of the band – Bryan McFadden left in 2004 – have been speaking about their reasons for wanting to call it a day.
Nicky Byrne, 33, Kian Egan, 31, Mark Feehily, 31, and 32-year-old Shane Filan told the Daily Mail the boys had simply run out of steam and lost the love for what they are doing.
"As a band, though not individually, I think we have lost ambition," Nicky explained. "As soon as your hunger goes, you have to face facts, man-up and make the decision."
Shane added: "We realised that we were not enjoying it as much as we used to.
"It wasn’t the record company’s fault — they offered us a really big multi-million-pound deal and we turned it down.
"We said we didn’t want to listen to any more offers. We could have taken the money and not delivered, but we wanted to be honest and end on a high."
Mark said: "I really don’t think any one person can be either blamed or take responsibility for the end of Westlife.
"There was no one incident that resulted in an argument and somebody stomping their foot and saying: 'Right that’s it. It’s over!'
"Everyone slowly started coming round to the same way of thinking, and we waited until we were all of the same mind before we took it seriously.
"Everyone makes up the band, everyone is responsible for the success and everyone is equally responsible for the end.
"Westlife has always been quite balanced like that. No one person has ever taken over and caused the beginning, the middle or the end of Westlife."
The boys all agree that there are no plans for them to reunite.
"We know some sceptics see it as a marketing ploy and think that we’ll make millions by staging a comeback in three years’ time," says Mark.
"After all, Boyzone and Take That re-formed after saying it was all over. But we have zero plans to get back together."
When they started out, the group were all teenagers, apart from Nicky, who was 20.
Mark, Kian and Shane, three friends from Sligo in the west of Ireland originally had their own band. Shane’s mother knew Louis Walsh and introduced them to him.
The X Factor judge brought in Dubliners Nicky and Bryan McFadden and in 1998 the group began their chart domination.
Since then they've sold more than 44 million records and had 14 number one hits.