Sarfardusa Hasti, 40, was shopping in store in Lombardy Shopping Park near Uxbridge Road when she felt something at her side.
She searched for her phone and found that it had gone and turning around spotted a woman rushing out the door.
The mum-of-two said she chased the suspect and called for security who followed outside, where Miss Hasti grabbed the woman, but the staff refused to intervene as the victim grappled with the alleged pickpocket.
Miss Hasti, who lives in Hayes, told getwestlondon : “The two security guards made their way outside but they didn't do anything.
“They didn't call the police. They said they're not allowed to search a woman.
“I and one other was holding the woman so they should have called the police.
“I'm more angry at the security guards than I am at the woman.”
Miss Hasti said the woman escaped with her mobile phone.
She added: “I'm a regular customer.
“They just stood there and did nothing.
“What if she had a knife?
“The store manager apologised.
“I said I'm never going to come back to your store any more.”
A Sainsbury’s spokesman said, “We are sorry for Ms Hasti’s experience and have investigated the incident with our security guard.
"While we appreciate this must have been very upsetting and stressful for our customer, it’s important to also remember that security guards aren’t allowed to physically detain anyone on suspicion of an act they haven’t witnessed themselves, and they are legally not permitted to search any member of the public.”
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