The former Westlife singers have developed a bitter rivalry because Kian, 33, was paid £40,000 more to take part in ITV's 'I'm A Celebrity...Get Me Out Of Here!' earlier this month than Nicky, 35, earned for appearing on the BBC series 'Strictly Come Dancing' last year.
A source told the Daily Star newspaper: "They're barely speaking; they're all looking out for number one now. Nicky was angry and hurt by the difference in their pay-outs as it creates an unhealthy competition among them - he wonders why Kian is worth so much more than he is."
The King of the Jungle took home £100,000 for his stint on reality TV, while Nicky was paid £60,000.
But Kian branded the rumours "absolute rubbish" on his Twitter page on Sunday (29.12.13).
Meanwhile, their other former band members, Mark Feehily, 33, and Shane Filan, 34, have also become very competitive with one another.
The source said: "Since the split there has been a lot of bad blood between the boys over their careers. Mark is hoping for big things next year and if it means casting a dark cloud over Shane's career then so be it."
Westlife announced they were splitting up in 2011 and performed together for the last time in June 2012.