The stars recreated Janet Jackson's famous 1993 Rolling Stone cover to raise awareness of cancer charity CoppaFeel! ahead of Breast Cancer Awareness month.
Porter posed with Hollyoaks colleague and boyfriend James Atherton, while the Spice Girl stripped off with her husband Stephen Belafonte.
"I was so excited to take part in this shoot with James," Porter said. "Janet Jackson's Rolling Stone cover is so iconic. But it's for a great cause too.
"Checking your boobs can be scary - you don't want to find anything. But it's so important you do as it could save your life. James and I make it fun - plus, I check his bits too."
Atherton added: "Raising awareness of any type of cancer is something that's close to my heart. It's a terrible illness and seeing someone you love go through it is devastating.
Mel B said: "I love boobs. Me and the other Spice Girls are always having a feel of each other's - Geri came up behind me and grabbed mine the other day.
"I found a lump when I was 17 and freaked out. It turned out it was nothing but it terrified me. Ever since, I've always been one for checking, then double checking. I have three daughters and one stepdaughter so I'll show them as they get older. My eldest [Phoenix, 13] does it already."
The October issue of British Cosmopolitan is on sale Thursday, September 13 and is also available as a digital edition.