Lily Allen said she has had a "glimpse of what it feels like to be discriminated against" after being shunned by a taxi driver.
The London-born pop star said she was turned away from a black cab after being told to "find an immigrant" to give her and her daughters a ride.
The alleged interaction between Lily and the unknown taxi driver comes after she apologised "on behalf of my country" after seeing the squalid conditions refugees had lived in at the Calais camp known as the Jungle.
Lily, 31, wrote on Twitter: "Just tried to get in a black cab with my kids. The driver looked at me and said , 'find an immigrant to drive you you stupid tart'."
She added: "Having lived a life of privilege, that interaction has given me a tiny glimpse of what it feels like to be discriminated against.
"There is so much work to be done. But we can do it. Let's stamp out this stupid horrible ignorance together. #takeourcountryback."
She was then asked by another Twitter user to reveal the car's registration and plate number in order to prove it had happened.
The mother of two fired back: "I had both my hands full with children, couldn't get to my phone fast enough."
She responded to another person who said that it "definitely didn't happen", writing that they were "victim shaming" her.
Lily declined to comment when approached by the Press Association.
She was criticised following her visit to the Jungle, during which she volunteered in a warehouse where donations came in before being distributed to the thousands of migrants and refugees living there.
In scenes that aired on the BBC's Victoria Derbyshire programme, she broke down in tears while meeting a 13-year-old boy from Afghanistan who said his father lives in Birmingham but who himself had camped at the Jungle for two months.
During their meeting, Lily told him: "It just seems that at three different intervals in this young boy's life, the English in particular have put you in danger.
"We've bombed your country, put you in the hands of the Taliban and now put you in danger of risking your life to get into our country.
"I apologise on behalf of my country. I'm sorry for what we have put you through."
Lily's experience came a week after Ugandan-born British journalist Yasmin Alibhai-Brown said she was refused a ride in a taxi.
She wrote on Twitter: "Last night, West Hampstead station, on my way to the Grunwick Strike event. Black cab driver doesn't let me in: 'Not taking foreigners today'."