Walker, 29, and a male friend are reported to have invited 21-year-old local escort Louise McNamara and a 24-year-old Brazilian call girl to his luxury apartment in Cheshire last week, where they spent three hours for an alleged lockdown romp.
Photos circulating on social media, taken by escort McNamara, show Walker stripped to his underwear and appearing to count out money for the call girls.
The next day, Walker took to social media to urge fans to follow government advice and stay home during the Covid-19 crisis.
City full-back Walker, who is said to rake in around £150,000 (US$185,000) a week, appeared to acknowledge the reports in an apology issued on Saturday night.
“I understand that my position as a professional footballer brings the responsibility of being a role model,” he said.
“As such, I want to apologize to my family, friends, football club, supporters and the public for letting them down.
“There are heroes out there making a vital difference to society at the moment, and I have been keen to help support and highlight their amazing sacrifices and life-saving work over the past week.
“My actions are in direct contrast to what I should have been doing regarding the lockdown,” he added.
Club bosses at Manchester City have expressed “disappointment” at the claims and are believed to have launched an internal disciplinary investigation.
Walker made headlines during the summer for his performances away from the pitch when it emerged he had split from girlfriend Annie Kilner after getting model Lauryn Goodman pregnant.