An upset customer has been left with a bad taste in his mouth after a pizza restaurant threatened to sue him for giving a negative review.

Julian Tully, a law student in Adelaide, wrote a review of Etica Pizzeria on the restaurant's Facebook and TripAdvisor page, after a recent dining experience.

'Stay away from this place. I went there with some friends tonight who are all seasoned foodies and were treated in a fashion that I didn't think was possible,' Mr Tully wrote.

An upset customer has been left with a bad taste in his mouth after a pizza restaurant threatened to sue him for giving a negative review (stock image)

'For 7 people we got a tiny amount of food (waiting more than 50 minutes in between the portions) and when we tried to complain in reasonable way we literally got told 'we have had our fill' and 'we shouldn't go out for dinner if we cant afford it'.

'Then they called the cops on us because we walked out. Avoid like the plague! (Unless you liked to be judged by a bunch of people who can't run a business in an 'ethical' way).'

The feedback didn't go over well with the restaurant's owners, who allegedly accused Mr Tully of defamation and demanded he delete his review and apologise for 'misleading' comments.

'This correspondence hereby puts you on notice that comments you verbalised on Gillies Street, Adelaide on October 10, 2015, as well as those published on 11 and 12 October, 2015, on various social media sites concerning Etica Pty Limited are defamatory,' a message Mr Tully received from the restaurant's co-owner read.

Julian Tully, a law student in Adelaide, wrote a review of Etica Pizzeria on the restaurant's Facebook and TripAdvisor page, after a recent dining experience (stock)

After sharing his review, Mr Tully claims he was contacted by the restaurant's co-owner who threatened to sue him

'Contents of the said statements are false, misleading, and direct malice towards Etica. Action is immediately required by you to delete published statements, and to issue an apology to the directors of Etica.

'Your actions reflect your poor character: such behaviour does not align with the values of a prospective legal practitioner.'

The message, which was sent to Mr Tully's personal Facebook account, also said he has until October 27 to delete the message, and that restaurant management will contact the Law Society of South Australia to inform it of his 'defamatory comments'.

The messages Mr Tully received reportedly from the restaurant's co-owner

Since Mr Tully posted his review, other people have also criticised the restaurant.

'Julian, I don't know you but I am on your side. They are trying to take away your voice but they will,' one Facebook user wrote.

'You suck.. nice lawsuit douchebag,' Matt Webb said, in a comment of Etica's page.

However, other customers shared positive reviews of the restaurant, which had an average rating of 4.2 stars.

The restaurant has mixed reviews online, with some people criticising it and others saying it has 'the best pizza in Adelaide' (stock image)

'Best pizza and customer service in Adelaide!' Sarah Elsayed wrote.

'Finally found the best pizza in Adelaide!' Andriana De Ieso said.

TripAdvisor ranked the restaurant as one of the top 200 places to eat in Adelaide.