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James, 44, came out as bisexual in 2009 and as gay in 2012. The first band member he told was Simon Webbe.
But he had previously told the mother of his daughter when she had called to tell him she was pregnant, after a one-night hook up years after they had first dated.
Speaking on White Wine Question Time, James told Kate Thornton about the phone call he got when he was in Ayia Napa with friends in 2004, describing it as 'probably the most sobering phone call I've ever had in my life'.
James said: "I didn't even know my own self. Everything was just so confusing. We were in the height of Blue, everything was a real blur back then, it was mad.
"I had a secret boyfriend. I knew that I was gay, but I didn't tell anybody. And then I find I'm gonna be a dad.
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"So it was just like: 'Hang on a minute, I need to tell this girl everything before she makes the decision.'
"I have to be honest with her. And that was just my own guilt, that I needed to be honest. So I just blurted it out. And then it was like: 'Oh, my God. I've told somebody.' That was that was the catalyst of me then telling my mum."
Claire Grainger and James had met before he rose to fame with Blue in 2001, when they appeared on the same episode of TV chat show, The Time The Place with John Stapleton, when she was in a Spice Girls tribute act and James was in a boyband called Volume Five.
The pair had dated for around a year, according to James, and then met up 'randomly' when he was doing a gig in Manchester with Blue.
"We had a few drinks," he explained. "And had a bit of a fun time together. And three months later, I get a phone call saying: 'I'm pregnant'.
"And I was like: 'Oh my God.' I was just going through the whole: 'Am I gay? Am I straight? Am I bi?' I didn't know what was going on with me. And then all of a sudden I get a phone call saying you're going to be a dad."
James, who also appeared as Ryan Knight in Hollyoaks, said he might not have had a child if it wasn't for Grainger, saying: "That might have never been a possibility for me.
"The gods aligned something up for me to have a child. I definitely believe that."
But he did not come out to his bandmates for some years afterwards, with Antony Costa saying he was 'selfishly gutted' that James had not come out to him first.
Costa remembered being on a date when James had phoned him late at night to tell him. He said he had to leave the date early because he was thinking of his friend, who he had known since they were young, and wanted to make sure he was alright.
He explained: "I was selfishly gutted that Duncs didn't open up to me first, because I'd known him the longest out of the boys. In my head, I was like, why couldn't you tell me last week, or a month ago, or three years ago?"
James said that as a boyband member, he was 'living in fear' of what might happen when he came out, because it had been made clear that they had to seem 'accessible' to their fanbase.
While Costa said he understood it now, he still asked why James couldn't have sat all the band down, including other members Simon Webbe and Lee Ryan, together to tell them.
James replied: "It's the pressure of actually confronting people face to face. When you've been hiding that secret for so long, it's like you feel shameful."
Looking back now, James said he didn't know why he had all those emotions and why it was 'such a big deal'.
"I just felt really frightened to be able to tell them," he explained. "Especially because I didn't know how they were going to react.
"Were they going to go and tell the record label? Were they going to tell management? Was it going to get out in the press?
"I just had in my head: I need to keep this really quiet. Nobody can know about this."
James said he was looking forward to Blue's forthcoming tour so he could enjoy it in a way he wasn't able to when he was younger because of carrying around the secret of his sexuality.
He added: "When I did my story, and I came out, it was the biggest relief. I look back now and I just think why was I carrying that rod for so many years?
"I look back now and I wish I had the opportunity to come out a lot younger."
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