The fast food chain is selling Whoppers in the US for one cent, but only if you order from its biggest rival's restaurants.
Customers have to download the Burger King app, and open it at their local Maccies.
BK promised it was "turning more than 14,000 McDonald's into Burger King restaurants."
Burger King has called the gimmick a "whopper detour" – but be warned, your local Burger King needs to accept mobile orders for it to work.
A secret deal hidden in the app only activates within 600 feet of any McDonald's restaurant.
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The chain said in a statement: "If a guest is inside one of these geofenced areas and has the new BK App on their device, the app will unlock the Whopper sandwich for a penny promotion.
It announced the promo on twitter last week, posting the cheeky message: "Brb going to McDonald’s."
The promotion kicks of today.
Some fans were baffled by the announcement, feaing Burger King's twitter feed had been hacked.
This week, we revealed one woman's hack to make KFC fries less soggy.
We also revealed that you have been eating burgers wrong for years – and here's how to do it properly.
And here are our tips for cheap buys at McDonald's.
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