The group said in a statement: "We're all over the moon to be breaking our own record.
The boy band first entered the Guinness Book Of Records earlier this year when their fifth single, Fool Again, went straight in at number one.
Their latest record-breaking release, a version of Phil Collins's 1984 song Against All Odds hit the top spot on Sunday.
The group's runaway success eclipses even that of their co-manager Boyzone's Ronan Keating.
Dream come true
Boyzone have achieved six chart-topping singles, but these were spread throughout six years of releases.
Fellow Irish stars B*Witched were the first group to have their first four singles go straight to number one.
The Spice Girls secured the number one spot with their first five singles, but did not enter the charts at the top.
Westlife's Against All Odds was recorded in Italy with American singer Mariah Carey.
The group said in a statement: "We're all over the moon to be breaking
our own record.
"Working with Mariah, our favourite singer of all time, is a dream come true.
"It's extra special for us to be breaking this record with her."
Carey originally recorded the song for her own latest album but after meeting Westlife in Los Angeles and hearing their work she suggested they collaborate on a new version.
During a break from shooting the movie All That Glitters in Toronto, she said: "Westlife have been fantastic.
"They were fun to work with and we just clicked creatively.
"I'm really thrilled that Against All Odds has gone straight to number one.
"It was always one of my favourite Phil Collins songs growing up."