"I'm like a stick of dynamite, light me up and I will blow your mind," she wrote alongside the image posted on Sunday.
Many commenters slammed her for disrespecting the memories of fallen soldiers.
"Pretty bad taste in front of a memorial for war dead,' one said.
Another said: "It's a cool photo but couldn't you have found a better wall to pose in front of?
"It just shows you have no respect for the people who gave their lives for this country."
Another also conceded that it was a "nice pic" but added "not sure backdrop is appropriate."
But others defended the singer, saying the photo was "artsy" and urged her to "ignore the haters."
Following the backlash, the mother-of-two, from Loughborough, hit back at critics, saying she "is pretty sure those boys that gave their lives for this country would enjoy a casual bit of sideboob."
But she also claimed she was unaware the plaque was a war memorial because she was drunk at the time, Nottingham Live reports.
The singer, who sang for the Queen at the launch of the BBC's Broadcasting House headquarters five years ago, added: "Me and my best mate Jonjo were being creative in Clapham and saw the church and he was like 'go topless.'
"That's it really. It's not that deep."
According to the paper, she added: "I didn't even notice it was a war memorial. We were drunk. I know people are offended, but why? It's a human body, why is that offensive?"
But alongside one of the images posted on Wednesday evening, she insisted she did not intend to offend anyone.
"Those names behind me, those boys died fighting for our freedom. I live in the free world they created and here I am, embracing my body and being free," she wrote.
"Take me back 100 years, stand me in front of those soldiers, what would they think? What would they say? What would they do?
"I think I look good in front of that wall. I haven't done it to upset anyone and I don't think it's a particularly offensive image, in fact anyone who checks my social platforms will have seen it before...
"Of course I'm not trying to disrespect the fallen. If I've offended anyone then it was not my intention."