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It is every shopper's worst nightmare - being stuck in a queue behind someone chatting on their mobile.

But it wasn't a shopper who finally cracked when faced with a woman chatting on her phone at the tills, it was the checkout assistant.

The worker took matters into her own hands by refusing to serve the customer unless she hung up.

The woman making the call, Jo Clarke, slammed the "rude" assistant, saying she had no right to "give her a lecture on checkout etiquette."

The 26-year-old was shopping at a branch of Sainsbury’s, in Crayford, south-east London yesterday.

After finishing her shop, she reached the front of the queue but the assistant told her: "I will not check your shopping out until you get off your mobile phone."

She said she wouldn’t serve her until she hung up.

The store has since apologised and insists it does not have a policy of refusing to serve customers who are using a mobile.

Ms Clarke, a property manager, double-checked with the customer services desk, which confirmed this is not the company’s policy.

She said: "I was dumbfounded at the time so just hung up. I don’t know what she was playing at. I couldn’t believe how rude she was.

"When did she have the right to give me a lecture on checkout etiquette?

"I won’t be shopping there again - I’ll go to Waitrose instead."

A Sainsbury’s spokesman said: “We have apologised to Ms Clarke and offered her some vouchers. It isn’t our policy to not serve customers who are using a mobile phone.”

But the incident has prompted an online storm, with the majority of people vociferously backing the shop assistant and criticising Sainsbury's for not standing by her.

"Good for the checkout girl. The store should have backed her to the hilt," was the comment from someone calling themselves "Termite", and "Good for the checkout girl, it's a pity that Sainsbury's didn't have the bottle to back the member of staff up", was another.

Tom Jones said: "Well done the checkout girl" and Martnal2 went further, saying: "Jo Clarke has... through her actions, inadvertently done so much to improve the manners of these mouthy phone pests".

Cherith Porter said: “I think that is a legitimate request that if you use your phone at a checkout they won’t serve you until you are finished.”

Another online poster said: “I think using a mobile phone when you are engaged in an activity involving face to face contact with another person is incredibly rude and offensive.

“Unless it’s an emergency put the phone away and try being polite and attentive to the person in front of you. Manners cost nothing.”

And another - known as HK - added: “It shouldnt have to be company policy. Common sense and manners tell you that you shouldnt be on the phone while you are being served.

"It’s just bad manners and ignorant as far as I am concerned.”