The former Love Island star confessed to being unfaithful during a “bad stage” in their relationship, saying he will “regret it forever”.
Somerville has written Allen an apology song which will feature on his upcoming debut solo album.
Speaking to London Live at the TV BAFTAs he said: “I’m busy focusing on my music. I’ve got an EP that’s coming out. There is a song about Gabs on there. It’s a sad song”
Asked if it is an apology, he replied: “Yeah, pretty much.”
Despite cheating on Allen he said she would still be his number one choice if he could take one person to a desert island with him.
He said: “I’d say Gabs – but she’d probably want to kill me while we’re on it.”
Somerville was accused of cheating on Allen with multiple women over the course of a few months, dating as far back as November. Last month the couple were rocked by claims he cheated on her during a romantic holiday to Mexico.
He confirmed the reports were true in a statement, telling Mail Online: “Sadly the rumours are true, Gabby and Marcel have split up. Marcel can't really defend his actions as he knows he did wrong and will regret it forever.
“Marcel is upset that he broke the heart of someone he loves and someone that he shared so many special moments with, but with all the good times there are difficult times, and sadly Marcel and Gabby had been in a bad place at this stage.”
He continued: “Gabby was a special part of my Marcel’s life, and he hopes one day she can find it in herself to forgive him.
“He knows what he did was wrong and is extremely sad about how he has hurt Gabby.”
The statement came hours after Allen thanked fans for their “love and support” after being left in “shock” by the revelations.