A retro restaurant serves burgers and milkshakes - while staff hurl insults at customers.

Footage shows ‘rude’ staff berating customers at a Karen’s Diner franchise in Sheffield, English. The staff, however, were only cosplaying as an obnoxious ‘Karen’ and were trained to give just the right amount of abuse to diners.

One of the ‘angry’ staff ever bearted a customer when they apologized for being late for her reservation. The worker said: ‘Oh a bit late? Don’t be sorry if it’s going to be like this sh*t next time alright?’

The customer could be heard giggling throughout the video as she had more staff interactions, each one staying in character while serving food.

Customer Kerre Chen said: ‘Obviously don't take kids there and it isn't really for people who are easily offended but you can always unleash your inner Karen back - in fact they encourage it!’

However, Kerre added that the staff seemed to have been trained ‘not to cross the line'.

She added: 'I personally didn't think they were that savage. [I] think they've been told to not cross the line, they don't make comments on your appearance - at least they didn't to us.’

Karen’s Diner also has restaurants in Melbourne and Sydney. They encourage customers to book through their website before dining, but walk-ins are also welcome.

They serve burgers, fries, milkshakes, and cocktails.