The fast-food chain is offering customers the choice between its signature burger, the flame-grilled beef Whopper, or the plant-based alternative.
The offer is available nationwide, and can be redeemed using the Burger King UK app.
Customers must register an account on the app, before visiting their local branch to claim their free item, with no other purchase necessary.
The offer will also be open to those ordering via Deliveroo, but these customers will need to spend a minimum of £15.
Katie Evans, chief marketing officer at Burker King commented: “The Whopper is Burger King’s hero burger – created using fresh ingredients, made to order your way and flame-grilled to create the signature, mouth-watering flavour that our customers love.
“For one day only we think everyone deserves to get involved with a bite of the action and savour a free Whopper on us. If you haven’t tried a Whopper before, here’s the day to try it.”
Burger King’s beef whopper, usually priced at £5.49, is made up of flame-grilled patty, topped with tomatoes, lettuce, onions and pickle.
The alternative uses a plant-based patty and vegan mayo. However, the patty is cooked on the same grill as the original beef burger, according to Burger King’s website.
Earlier this month, Burger King’s German arm caused a stir on social media after debuting a promotional menu for Mother’s Day at one of its Berlin restaurants, which included burgers layered with ice-cream, strawberries and whipped cream to name a few.
The campaign, dubbed “The Pregnancy Whopper” was intended to appeal to expectant mothers and the bizarre pregnancy cravings they may experience.
It comes after a survey of 1,070 pregnant women and mothers found that 76 per cent believe pregnancy cravings are impossible to resist.
The burger maker said it had created the special-edition Whoppers using some of the most popular food combinations according to the survey, such as cucumber and jam, fish sticks and applesauce and curry sausage and fried herring.