Diners at a trendy Shoreditch restaurant were appalled when they were charged £2.99 for "the privilege" of paying the bill on the owners’ own app.

Customers visiting Italian eatery Gloria discovered the 'checkout fee' on their digital receipt after consuming a meal of £193.50 on March 28.

Alongside the regular 13.5 per cent service charge of £26.15, they were slapped with the additional fee, taking their bill to £222.64.

The 'checkout fee' of £2.99 is a charge only for those who pay via the mobile app named 'Sunday'.

This app was developed by French entrepreneurs Victor Lugger and Tigrane Seydoux, who also happen to be the co-founders of The Big Mamma group that owns Gloria.

Avoiding this extra charge is possible only if customers ask to pay via contactless card or cash.

However, they first have to get hold of "notoriously slow" staff to make this request.

This Is Local London: The Italian restaurant uses the Sunday app to speed up paymentsThe Italian restaurant uses the Sunday app to speed up payments (Image: Google Maps)

The fee sparked a public outcry with people taking to social media to express their frustration.

These diners also left one-star reviews on the restaurant's page in backlash.

One Twitter user said: "It actually borders tacky.

"People are sick to the back teeth of booking places, bonkers service charges and still expecting tips, and now stupid things like this."

Another user added: "This needs stamping out fast before it spreads.

"I hope the restaurant gets boycotted until they see sense."

The Sunday app, launched with the intention of saving diners time while settling their bill, allows users to scan a QR code and divide the payment with friends.

Despite these user-friendly features and two million businesses adopting it since it was created in 2021, the app has a two-star rating on Trustpilot, with 79 per cent of users rating it one star.

App user Natalie Morgan said using the app was "daylight robbery", and that it "fleeced" her out of money on an "already expensive bill".

She added: "The staff in Big Mama Group are notoriously slow and avoid you when it comes time to pay, so you have to be quite determined to get hold of them."

A Big Mamma group spokesperson said: "Big Mamma does propose Sunday as a payment option for clients who wish to pay faster."

However, they clarified that the app is "100% optional" and "more than 60% of guests" still use traditional ways to pay, free of charge.