He made the admission during the filming of 'Made In Chelsea End of Season Party', adding that she had dumped him as a result.
Open Image Modal
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."
Open Image Modal
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.
Ashley Cole became public enemy number one when it was uncovered that he'd been cheating on his wife, and the nation's sweetheart, Cheryl Cole in 2008 with a hairdresser. But even after a string of other women came forward to say Ash had bedded them too, Chezza still took him back. But their marriage came to an end in 2010 after a tabloid exposed him as having done the dirty again. (credit:PA)
Hugh Grant(02 of14)
Open Image Modal
Hugh Grant stunned us all when he cheated on then girlfriend Liz Hurley with Los Angeles street prostitute Divine Brown in 1995. Unfortunately for him, he was busted by the police as well as by Liz. (credit:Getty Images)
Mc Harvey(03 of14)
Open Image Modal
MC Harvey left wife Alesha Dixon devastated when he cheated on her with 'Popstars: The Rivals' flop Javine Hylton. The couple went on to have a child together, but later split when Harvey was unfaithful again. Well, a leopard never changes its spots... (credit:PA)
Mark Owen(04 of14)
Open Image Modal
Take That star Mark Owen has confessed to having affairs with 10 women since he began seeing his wife Emma - almost putting him on par with Tiger Woods in the infidelity stakes. (credit:PA)
Wayne Rooney(05 of14)
Open Image Modal
Footballers these days seem virtually incapable of keeping their trousers zipped up and Wayne Rooney was no exception when he allegedly paid hooker Juicy Jenny £1200-a-night. They reportedly met up for up to seven romps, all while wife Coleen was pregnant with baby Kai. (credit:Getty Images)
Tiger Woods(06 of14)
Open Image Modal
Golfing legend Tiger hit the headlines in spectacular fashion in 2009 when a string of women came forward claiming to have had affairs with him. (credit: AP)
Vernon Kay(07 of14)
Open Image Modal
Vernon Kay has confessed to sending sexy texts to a number of different women, leaving wife Tess Daly furious but still standing by him. (credit:PA)
Jude Law(08 of14)
Open Image Modal
Jude Law was exposed as cheating on Sienna Miller when Daisy Wright sold her story to the press. He got dumped for his troubles, but still Sienna took him back a few years later. (credit:Getty Images)
Brad Pitt(09 of14)
Open Image Modal
Brad Pitt has always denied cheating on Jennifer Aniston, but he was heavily rumoured to have been hooking up with Angelina Jolie while they filmed 'Mr and Mrs Smith' together. (credit:bauergriffinonline.com)
Rupert Sanders(10 of14)
Open Image Modal
It was the affair that rocked Hollywood - director Rupert Sanders did the dirty on his model wife Liberty Ross and broke up one of the most solid couples in celebville in the process when he had a secret liaison with Kristen Stewart. (credit:Getty Images)
Gareth Gates(11 of14)
Open Image Modal
Gareth Gates was alleged to have been conducting an affair with his young 'Les Miserables' co-star Katie Hall, while his wife Suzanne was at home looking after their young child. (credit:PA)
John Terry(12 of14)
Open Image Modal
Chelsea footballer John Terry lost the England captaincy after it emerged he'd cheated on his wife Toni with the partner of his former team-mate Wayne Bridge. (credit:PA)
Zayn Malik(13 of14)
Open Image Modal
Zayn Malik broke many Directioners' (and girlfriend Perrie Edwards') hearts when an Aussie waitress claimed to have bedded the boybander after a party at his home, and even had a snap of a sleeping Zayn to prove it. (credit:Twitter)
David Letterman(14 of14)
Open Image Modal
David Letterman confessed to a string of affairs with female colleagues after he became the victim of an alleged blackmail attempt in September 2009. (credit:AP)