He made the admission during the filming of 'Made In Chelsea End of Season Party', adding that she had dumped him as a result.
Spencer and Lucy during happier times
He said: "I've slipped up. I've cheated on her. It was pretty recently. A couple of days ago. I was in Greece. I think I'm just young and living some form of dream but unfortunately hurting people in the process."
The couple had only been together a couple of months, getting together after he ended his relationship with Louise Thompson, who he also cheated on.
At the time, he said he was a 'changed man' and wouldn't cheat on her as he doesn't do it to women 'he respects'.
He added: "If anyone could have changed me, I still think it could have been her - could still be her. I don't know. I'm in a bit of a predicament at the moment. It's a horrible thing to do to someone who deserves it as little as Lucy."
Spencer previously cheated on ex-girlfriend Louise Thompson
Having her say, Lucy admitted she couldn't even look at her ex-boyfriend after his dirty deed.
"I found out because I got tweeted. There are some nice people out there looking out for me. What kind of person DOES deserve to be cheated on?" she said.
"He denied it for about 15 minutes on the phone. I got a bit upset and broke down. He managed to admit it over text, which was very brave of him."
After Spencer insisted he was actually 'very upset' about the situation, Lucy hit back: "You're selfish, that's the problem. Can I just say, that after this is finished, please do not ever speak to me ever again."
Safe to say that the next series of 'MIC' looks set to be as explosive as the last...
You can catch the 'Made In Chelsea End Of Series Party' on E4 on Monday night.